You can't remove the forumid - vb needs that to know which forum to show

As for removing the 'f/' Its doable, however the more you mess around with the internals of the rewriting the more issues arise from it.
I have a really nice bit of code that rewrites threads to be /forums/forum-title/subforum-title/thread-title-threadid as elegant as it was, it required lots of little adjustments here and there to have it work properly.