Some vendors have blocked prepaid cards. Virtual or physical, does not matter for this, both are usually blocked and your payment may not be accepted in these sites. For example in sites like Upwork.
A: Volopay recommends to do it via Paypal, which will be more secure and hassle free in the above situation. Kindly follow the below steps to make the payment successful:
Create an SG based Paypal account (business or personal one)
Tie your Volopay virtual or physical card to that Paypal account
On the payment site, use Paypal to make the payment
Below are images used to make a payment in Upwork through Paypal and Volopay card:
B: Card tagged as fraudulent while making online transaction?
At times vendors also blacklist prepaid cards on their websites, due to which the cards are not accepted on their website. You can simply get in touch with the merchant and get the cards whitelisted on their website to make all the future transaction to go through.