what is the purpose of a VAC gauge?

i was given one as a gift and i want to know more about it
and where can i find the gauge pods for my 91 accord that are mounted like this




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oops, wrong picture, i meant this one
http://dragonautomotive.com/sp-bin/spirit?PAGE=175&CATALOG=24

Check eBay.  Thats where I got mine.  I paid 30some bucks and the paint is almost perfect.  But at the bottom where it is between the dash a little has rubbed off. Aw well.

yeah, but i couldnt find the A-pillar one for my 91 until i looked at this site
http://www.jcwhitney.com/autoparts/Browse/tf-Browse/s-10101/N-111+1991+200728704+600010337/c-10101

but what is is the VAC gauge for?

The VAC gauge I am pretty sure just tells you the status of your vacuum.  To much or to little means theres something wrong.