Did you read out the full flavour text? If so, recovering that money is part of what they were hired to do.
I presume they at least managed to get the signet ring? If not, her reaction should be "get back ut there and finish the job!"
If they did recover the ring, and she knows they've got the money and they show no signs of giving it up, have her act disgruntled and say "Well, you seem to have already claimed your payment." 100 pp is enough to cover the payout she offered them if it's a party of five characters, and if there are more or fewer, well, close enough. They're off to a bad start, though - maybe reduce the monthly salary she offers to 80 gp.
I would say, as Lavinnia is written, she would be very unhappy with the characters. She certainly won't be trusting of them with sensitive information and that could fundamentally "break" the adventure if you plan to use it as written. A lot is going to depend on how the inter-party struggle between the "goody-two-shoes" and the members who wanted to keep the money plays out.
The way I see it, you have two possible routes you could go.
1. The party realizes that it might be bad to steal from their employer and turn away from their path of greed (well unless no one notices the rogue pocketing some coins here or their).
2. The party goes mercenary and someone or something else motivates them through the rest of the adventure. (ie: Rowyn, another DM created NPC).
Either way, I would work hard to make Vanthus as much of a villain as possible to unite the party on at least that common goal. However, my group started like yours and when Rowyn made her offer, they switched sides. I don't know the make up of your group so I can't help you past that. At least my PCs all decided to take the 100 pp without even discussing it. ;)
Hope this helps.