everyones main concern with vasari is that it moves, so this is the easy counter. use its massive size, slow movement, and horrendously bad turn radius to your advantage.
seeing as to how easily it gets hung up on resourse mines, structures, and ships. use these to your advantage. jump in with a small "decoy fleet" or even just a scout (something fast) LF are a good bet. lead it to the other side of the gravity well, behind a wall of structures, or even behind the planet itself. then jump in your fleet and you SB builder. while the enemy SB is hung up on structures or too far a way, shred his repair bays and jump inhibs with your fleet and siege cruisers, build your own SB. i often jump in 2 SB builders so if they kill my first attempt ill have another better positioned away from threats (if i afford that of course)
idealy you have plenty of bombers to kill all his military structures off while playing ring around the rosey with SB (usually dont wanna kill the civ structures because they will be serious stone walls if you stear the SB into them just right), just run your fleet around hitting SB with bombers if they havent been all flak'd out and if your vas run your SB while you upg it as needed. frontal deflectors is a must if you use vas SB vs another SB. you can bomb a normal planet with a marza or evacuator lvl 6 before the SB get to you, so if you can capture it and build repairs opposite side of planet from SB your money.
this is a basic outline, but bottom line is that with proper micro you shouldnt need to engage vas SB until your ready. with experiance youll find its much easier than it sounds