You can change how much credits are worth in bulk or single amounts at the buttom of the vBCredits Global Settings, in the admincp - vbulletin options area.
Example: $1 = 1000 credits, and also select the currency .. example if your using paypal .. CAN US DE etc, we have more info on that page for the exact way to set it up.
You can also define for example that if someone wants 0 999 credits for day .10 cents a credit, and then say between 1000 to 2500 credits, that a user donates for could be 0.08 cents, for buying more.. the script is really flexable.
so for your question above you prob just got the script installed but haven't changed our default settings yet in that side of the hack.
I hope that helps a bit. If its bug, and your still having issues, we have a bug tracker setup on
www.vbcredits.com to report any issues, and we'll get them fixed asap.