i have quite a bit of experience in dealing through ebay. some of the things mentioned are not totally correct.
Q. can you use your vpost address when bidding to ship to US only bids
most of the time the answer is NO.
accepting non-US bids is a separate issue from shipping outside the US.
this depends on whether the seller allows mail forwarding, or mailing to a PO box (which is what vpost is).
many sellers will not ship to a mail forwarding location. furthermore, many sellers will not ship to a mailing address which is different from the billing address.
even if you can change your ebay location to USA (im not even sure if that is possible), this only concernes your shipping/mailing address. your billing address will be determined when you make payment. i don't know of any form of payment other than paypal.
where paypal is concerned, firstly, you cannot change your location (across countries). if you have a singapore paypal account, changing your location to USA involves opening an entirely new account. the difficulty is in linking your bank a/c or debit/credit card to the paypal a/c. you need to do this in order to make payment.
linking your bank a/c or debit/credit card to paypal becomes a problem when you are in fact not in USA.
but even if you could get the mailing/billing address issue sorted, it still does not answer the very first question - whether the seller is willing to ship to a PO box or mail forwarding address. if not, and you win the bid, he will find out eventually and the deal may break down, and consequences may follow.
do not proceed with the bid if the seller may assume you are located within the US. make sure he is clear with this.
the reason why it is so complicated is obvious - with all the hoops you have to jump through, there are many vulnerabilities and many ways in which scammers can rip the seller off.