Credits are formerly known as vdollars, or Virtual Dollars.
They're used for VBetting on tennis matches (look in the tournament sub-forums, then at the top you'll find another sub-forum for VBetting)
It works just like real money, but you can't keep it
You get more credits for making posts, starting threads, editing posts, making bets and winning... You lose them if your post/thread gets deleted for one reason or another, or if you go a really long time without posting, or if you bet and lose it.
Here is the Sub-Forum for Virtual Betting Indian Wells:
You can click on threads and bet on the matches (although there aren't any open bets right now)
And here's a post I found by a moderator that may help:
Originally Posted by ceze
As you may already know, vCash (virtual Cash) is paid to valid registered account holders on WTAworld.com for the positive contributions they make to the community as a whole. You can earn vCash for a range of different contributions including: posting messages and threads, when other members view your threads, sharing your digital photos, posting blogs and much more!
$3.00 paid per new thread (except in non tennis)
$0.02 paid per thread pageview (except in non tennis)
$0.25 paid per new post (except in non tennis)
$10 per personal blog entry (once every 24 hours)
WTAworld.com Blog Publishing Community
$10 for logging into your blog account (Maximum: 1 bonus per hour)
$150 for every blog entry posted (No limit; reward paid for every blog)
$25 for every interview transcript posted (No limit; reward paid for every transcript posted)
$10 for every 'did you know?' fact you add to your blog database (No limit; reward paid for every fact you share)
$25 for posting the latest result on your blog (Reward goes to first member of a team who posts the result)
$3 for reading and commenting on blog entries in our Fan Blog forum (http://www.wtaworld.com/forumdisplay.php?f=664
$5.00 for every photo you upload (no limit; each image contained in ZIP files/archives uploaded are correctly counted as multiple image uploads)
You can "risk" your vCash by trying to accurately predict the outcomes of tennis matches in our virtual betting game. Visit: http://www.wtaworld.com/vbookie.php?