A lot of people use the bastion and call it a VSG. Just put a little something different on it if you have time.
As for how it works.
You pay 100pts for a 3 shield VSG.
The shield creates a 12 bubble around the building and everybody within the bubble is protected by 3 armour 12 shields. When the enemy targets a unit within the shield they hit the shields instead and have to glance or pen all 3 shields before they can shoot the units within them.
Grav and haywire does not work on the shields either.
You can good VSG rules for better details.
Also ITC has an FAQ that will change a the way the shield works a little bit.
There are two things that I love. Kicking ass and chewing bubble gum... And I'm allllll out of bubble gum!