Please note that US PSN have much more meaningful sales than MY PSN. MY PSN is always more pricey except the exceptional few. If you look at February Free games, MY gets  some games offered in January (no English subtitles).

Best seller for US PSN prepaid is:
hazwandek and wierdoz

Yes all digital game price is in MYR. To a certain extent US PSN has better deals and better offers but MY PSN has improved a lot vs when it started out. There are still price gems vs US PSN for some Vita games (just a couple of months back, games like Little Deviants, Resistance Burning Skies, Motorstorm RC are all cheaper than the US PSN as well as card games). Now they also have Dragon's Crown at RM 81 which is RM 50 cheaper than the card game and about RM 20 vs US PSN. When Killzone Mercenary was launched, the price is also the same (or slightly cheaper than US) though US PSN did have an offer for USD 9 for KZ Mercenary recently.

Just in case you think I'm pushing PSN MY, do know that I'm a PS+ member for US, UK & MY, and I'm a dead bargain hunter when it comes to online purchases so I would compare easily across the regions. That said you can't go wrong with subscribing either one so if you think you want to set your account to US is ok. I'm advocating more on MY coz I think with more support shown here, they will improve further. Plus they will have one advantage US & UK will not have which is access to Japanese games (free or discounted) but only if you're into it of course.

Can but he need to set up your account in the other vita to access the games you bought. So he'll be playing on your account. No issue if mainly offline stuff.

Only con is just that using ur PSN account instead of his own loh. If he wants his own trophies then need to buy separately.

Not so sure but if you do it in ad-hoc mode it can be possible.

yup definitely! eventually alot of games will go into PS Plus once, odds are if the one u want just got into another region's PS Plus and a matter of time.

but yeah, it would be a waste