Essentially, I have the opportunity to obtain a game voucher for a couple of PS Vita games that only allows me to download from the UK marketplace. My PSN account is based in New Zealand. What I am thinking is to create a UK account, use the voucher, download the games, switch to my main NZ account and play the downloaded games. This all sounds rosy, but is this really plausible?
No, this will not work. You can only have one account from one region active on a PS Vita. This is the same with the PSP, fwiw.
To change accounts you actually have to reset the device, wipe the content and activate the alternate account with its own SD card, as the memory card content is locked to the account it was initialized to.
Oh, and a word of warning. Sony limits the number of activation/de-activations per account. Once you hit that limit you can't switch the device again with an account until the window resets (I think it's currently one year).
Sorry for the bad news.
For better or worse, this has been done ever since 2006 when the PS3 was released. Back then people who bought the PlayStation 3 and did not have a "local" (Their country) PSN, had to create accounts with "false" data in order to play. That's the case for a lot of people in Latin America (in my experience) who have their accounts as US accounts, even if they are from another country.
Focusing on your question, I haven't tested that specifically with the PS Vita games, but it is perfectly doable with PSN games, you can download games from another region with a different account and then play it with your "main" account. A good example is with games in the Japanese PSN, there's lots of games that take forever to be released in other countries.
It'd be interesting to investigate if this is posible in the PS Vita since it only allows a single account.
I know this may not answer your question but I hope it helps.
One word of warning, in the PS3 at least, DO NOT delete the account you created to download the game, it renders those games unusable.