This API plugin is total crap. I am trying to work out a resolve with SiamPay now. This is a very very bad module to use. After you customer inputs their credit card information and clicks to confirm their order, your customer will see an order confirmation page saying thank you. At the bottom, there is a small button that reads "SUBMIT". If the customer does not click submit, you will not charge the credit card, but virtuemart still send you an order request. If they do click submit, then they are REDIRECTED to SiamPay.com and asked to enter their credit card information AGAIN, just like PayPal.
I also feel it is important to stress the amount of confusion on the order confirmation page. Your website will say "Thank you, your order has been placed" but there is absolutely nothing to inform the customer that they must click on "Submit".
This is NOT an API plugin for a "PS_AUTHORIZE" request.