Using VB for the client and C++ for the middle/server tier is a common model. C++ is used for performance, among other goals, but it takes a more knowledgable staff, which of course means higher salaries and hence higher costs. If you have the right talent alot can be done in a short period of time. If, as it seems, you don't have someone of that level, then of course it will effect the time schedule while they learn. Without knowing any details of your project it's hard to make an evaluation about technology. If you trust your team, you should listen to them. On the other hand is this person just wanting to improve his/her skills at your expense then jump to the next higher paid position.
