Assigning role based on certain input Parameter values

My requirement is

when the form submitted, based on input parameter value "X", I have to assign the user for my action/role..

For an Example:

Form -> Vendor Parameter value= V1 or V2

If v1, in the visual approval Callable Object, if the Vendor value = V1 approver would be A , if the Vendor value = V2 approver would be B.

I can understand calling the users management.. but how to do the business logic before assigning the users?

Please help me on this

Thanks

Gopal