I'd really stick to the programming language you're most familiar with, in this case VB. If there are issues with the programming language (ie., "too slow"), then your experience can find workarounds for those. That would be easier than learning a new language from scratch.

As noted by others, there are also some existing VB Gnutella clients around (one of the first clones was written in VB, and many followed). Check out http://www.gnutelladev.com for some clients with published code, including a handful of VB versions.

-- Mike