There's two situations when you are Vasari and the other player is ANY race.
1) You take out the fleet first, ignoring the starbase, and when the enemy fleet is gone, you either attack the starbase outright or use your own SB.
2) The other guy has a massive fleet and SB. You build however many Kostura cannons you can, and fire all of them at the planet (or you could fire one somewhere else and skip the planet altogether). Anyway, with about 7 shots, you kill all fleet and structures at the planet as well as any non-upgraded SB. From there, it should be easy to build your own. BTW, staggering shots will help keep the SB dormant longer.
And to all you naysayers of option #2, I have done this multiple times in Entrenchment. Entrenchment allows you to get a crazy econ going, and once that happens, 10-15 Kostura's is not out of the question. There's only one level of research and so therefore, TEC and Advent won't have time to react.