Hi Mike
we do this type of work
there are many questions to ask jyour client
1) what are they using for a payment gateway, if they are using americart, they will need to get a new one that is compliant with VM
2) shipping and payment methods, if they are using americart, they will need to get a new one that is compliant with VM
3) affiliate program if they are using americart, they will need to get a new one that is compliant with VM
4) wishlists, referral, share with friend, ratings,
5) related or last viewed products
6) quickbook integration
7) template design new template or build from scratch. (bootstrap)

can the customers, users and orders be exported from americart
9) qty discounts and shopping groups
although this is not the whole list, you need to find out if any of these are currently in use, and then fins the vm equivalent
of course it is different, and the customer has to accept that vm works the way it works. and you don't want to do a lot of customizations
this needs to be made clear set the expectation
we call getting all of this information phase1, and should be a part of the project scope
i hope this helps. cart migrations as long as we can get at the db, is no issue