This is already included in vbcredits.
visit Currency Pricing Conversion in the vbcredits admin menu which will take you to set up a payment processor if you havent yet, otherwise you set up your conversion prices for purchasing currency. then if you have a purchase event set up in vbcredits (included by default) then users can purchase credits anywhere the amount is shown from the currency popup. new in v2.1.0 is the ability to include a link to purchase which opens the popup directly and makes it more obvious for your users.
let me know if you need more help