DM advice needed, please help

Hello everyone.
I am a relatively new DM, running IRL campaign for my friends (now switching to online).
I have a situation and I am not sure what would be the best way to deal with it, I am hoping to get advice from more experienced DMs and also players who wish to give their perspective.
The situation is like this. The party is 5 players: two mages, ranger, bard (muliticlassing to rouge in near future) and a fighter.
The bard has been given "an offer he cannot refuse" by a local mafia boss, to infiltrate a masquerade party at a noble estate, break into the vault of the owner and steal everything not nailed down.
There was a list of specific items that had to be retrieved.
The party has an informant NPC who will be going with them and provide a way out of the vault.
The vault itself I had planned as a sort of dungeon crawl through a museum, the owner is a collector and he has been organizing his collection to be displayed to guests.
The party has already done a lot of research and preparation for this, so the next session will start with them going to this masquerade party.

One of the mages has been preparing to help the party with infiltration and provide distraction if needed, but she said specifically that she will not be going into the vault.
This makes perfect sense for her character to not get involved with any sort of criminal activity, even though she is prepared to help the party any other way she can.
Now, I have a problem with that, because I think it would be lame for her to sit doing nothing for most of the session, while her friends have all the fun in the vault.
I have been thinking what can I do to have her have fun as well.
Here are my options so far:
1) talk with the player out of the game and explain my expectations, e.g. tell her to be prepared to sit idle for most of the game if she does not which to go into the vault.
2) send her a vision in the game. Her character has a lot of visions and prophetic dreams, she regularly casts augury, that was the whole deal for her character.
So in principle, I can sent a sort of message to her character in the game, saying that her friends need her help. So far she has been following up on all such visions, but I don't want to abuse this mechanic.
3) Deal with the fact that the party will be separated, and switch in-between the "main" party going through the vault and the mage doing social stuff at the party.
I have several NPCs with important information and some fun encounters she can have at the masquerade by herself.

Ideally, I would like to let the whole party know that the vault will be a big thing with enemies, traps, etc, and not just one room with stuff, to hopefully motivate all characters to join.
I think I have done it some extend, but you can never be sure how your players see things.
I do not have a balance problem with enemies, I have already prepared for a party of 3,4 or 5 (the fighter may or may not be there because of the player situation IRL).
My main concern is that everybody are having fun.
It is especially important for me this time, as we have not played for a while because of life and corona, this will be our first session online, and the mage player had not played with us since she had a baby, I want to give her a good game.

Any advice and thoughts are very appreciated,
Thank you all in advance.

I would talk to the player out of session, but rather than say 'if you don't go in you will be bored', ask here if she has any ideas of how she can be involved while seperated.

I think you already have the idea, but she might be able to use Sending to communicate with the rest of teh party at key times. Maybe she needs to watch for when the guards start their rounds, or when the owner does X, y, or z. Maybe she needs to distract or delay someone?

Doesn't directly help, but The Alexandrian has some good tips on running heists as well. Though I can't find the link to the exact article at the moemnt and short on time. If interested, you can start looking here and maybe you can find it quicker than I could :)
https://thealexandrian.net/wordpress...1-the-villains

One way I run divination in games is to retcon scenes. Because you can't really tell the future in order to give the player advice before you know what will happen, I do it after the fact. Here's what I would do:

Give the Wizard a number of "interventions" based on their Int bonus, and level and +1 for any "higher" level spell slots spent on divination (top two levels of available spell slots regardless of character level). Each intervention allows them to have done something "before" the team goes into the vault. But you actually resolve it on the fly, as an interruption to the encounter. In D&D 5e a reaction, for example, I don't always remember the words for other systems. But as soon as you describe a room with 5 guards, let the Wizard say "actually, I tried to give three of those guards food poisoning so they might not all be on duty or at full strength" and then you quickly resolve that scenario. Then you apply the results of their precognitive intervention. Other examples my players have used in play were weakening the straps on a boss' suit of armor, swapping out an alarm horn for one that won't make loud noises but looks identical, making a copy of a key and giving it to the party before they leave, and distracting a patrol so it is 2 minutes late on its route.

In each scenario, they can have 1 PC help them but cannot duplicate PCs until they team up with everyone at least once. Resolving it is usually a bunch of skill checks or whatever, sometimes there is a really obvious way to make it happen that is so easy you think the character could achieve it automatically. Others are super hard and the character might end up hurt or captured.

This keeps the player involved and paying attention to the flow of the encounters because they are looking for a chance to do something cool. And they can partner up with other players for the distraction so they have more flexibility in what they are capable of doing. And it takes resources from the party, which is important for maintaining encounter balance in most game systems.

I know you might not want to hear this but I don't run scenarios that exclude a PC character. I don't think it's fun.

That said - what can you do? You do a bait and switch scenario. The idea is figure out a scenario where the party isn't *really* working for the mafia boss, but for the non criminals. IE the mafia boss' right hand man is an informant, so the local authorities know what's going on. Knowing what's going on, the authorities reach to the PCs to help the take down the boss, when the boss shows up to collect the loot from the list (the boss doesn't trust his underlings with that type of loot/information).

The basic idea is on the surface the job is exactly your current scenario, but the "real scenario" is something your mage PC would be happy to do and you have to add an extra boss fight for the mafia boss. And if the boss actually gets away then you have a nice recurring villain.

Why wont the mage go into the vault?
Are her expectations of what your game is different to reality?
Are her expectations of what D&D is different to reality?
Are you running the wrong adventure for her or in the wrong setting for her?

I am a fan of simple solutions, so I personally would offer to give the mage a simple-to handle pregen to play during the session that joins the heist.

Edit: If it's only the character, not the player that doesn't want to break in.

Have a talk with the player, she may be fine just watching the heist. Or offer her to play the NPC informant going in with the group.

Overview with a magic mirror, maybe let them cast minor spells through it?