(2) Yes. See no problem there. But, it may not be as easy as using vb.net with sql server
Disclaimer: This is based on my personal experience only.
(3) The perf lag is common irrespective of whether it is db2, sql server etc etc . Depending on that, you design your apps in such a way that comm over the network is minimal. As said in (2), I see no reason not to use vb.net with db2.
Use the search features in the following websites. Should provide you most of the info you will be after .. If nothing helps, of course, post here!