How to buy free game?

In specific response to the details of your question there are two answers:

1) No. In that you would have to cancel your PSN+ membership, but cancelling the membership blocks you from the discount. If you renew your membership you may regain access to prior downloads, which would again block purchase since it is already "purchased". (I suppose there is a sketchy chance that you'll loose all your old licenses if you renew a cancelled acount, so in that untested case this might become a Yes).

2) Yes. Create a new PSN account with a new PSN+ subscription. Wipe your Vita clear and loose all of your old user data. Add the new PSN account and purchase the game. ...the side effects of this approach in my mind would equate to a No rather than a Yes, but it's all about how desperate you are.

Personally, I just went and bought the physical copy. Once it's patched to the same version as the PSN download. You can transfer your save data between copies on a PC from the Content Manager backup folder.

How to Access Your Purchased games on PSvita

Go the the Playstation Store and on the right hand bottom corner is a little bubble (...), click on that, a new menu will flyout with the option of "download list". Here are all your downloads/aka purchases, you can download from your entire collection including games you "bought" when they were free from Plus.

What Happens to your store view when you buy an IGC "Free" PS+ title

"Purchase" doesn't show up for you anymore because you already own the game. Take for example this screenshot of my account, I "bought" Yakuza 4 while it was available for Free as a IGC, I still have PS+ membership, when I visit the store, even on the web, the ability to "purchase" is absent and only the ability to "download" - this is a bit of evidence that shows you have to NOT be a PS+ member to get the option to "purchase" an IGC title you "bought".

If you do not renew your membership, your access to download "Instant Game Collection" games will be revoked. It's not 100% clear from the Playstation Plus FAQ, but based on this question/answer we can see that you have the option to purchase the game but it seems to be only AFTER your membership expires:

Q: What happens to the games and Add-Ons I downloaded free through a PS Plus membership, and I decide to not renew my membership?

A: If you do not renew your PS Plus membership, you will not be able to access previous free games downloaded with Plus. If you have purchased Add-Ons for a game you received free in Plus, and it requires said free game to play it, you will not be able to access these Add-Ons if you choose not to renew your membership. However, if you decide to re-activate your membership, you will once again be able to play all games downloaded from the Instant Game Collection along with all purchased DLC for these games. Alternatively, you may also purchase the specific game to access any purchased Add-Ons for the game as well.