Well that all depends on what type of vampires we're talking about here. Are we talking Marvel-style, where any sort of UV radiation will hurt them, or are we talking WoD-style, where it's a mystical quality of the sun that inflicts damage on them. Now, if it's the first one, he should be able to simply defend his mutant garlic farm with a series of UV lights placed at overlapping intervals. If it's the second one, I'd recommend he find a local Etherite or Virtual Adept cabal to set up some equivalent device, only instead of UV lights, you'd have to use a Correspondence effect (probably disciple level at least, more likely adept) to co-locate the sun's rays to the tower, so as to ward off vampires again.
Of course, with the increased radiation from the wi-fi tower AND the solar rays, this will probably cause the garlic to rapidly evolve into some advanced life form heretofore unseen.
I, for one, welcome our new mutant garlic overlords.