When purchasing an SD card for any of your devices not just the Vita you need to be careful as there are a lot of convincing fakes on the market.
A fake SD could for example be sold as a 512gb Sandisk and even tho it looks real and the packaging looks real, in reality it could be just a 16gb card thats been spoofed into reporting as 512gb and rebranded
A good first rule is to stick to buying from trusted stores and avoid places like Ebay, Facebook, Aliexpress and others like them. Also stick to brands like Sandisk, Integral, not ones with random names or stores own brand cards.
If your buying from Amazon then make sure it says 'Sold by' and 'Dispatched by Amazon'
How to tell if my SD is Fake?
There are a few apps you can use to test an SD cards true capacity, but there is also a fairly simple way to check as most fakes currently on the market miss report their faked sizes.
If your able to put the SD into a windows computer, right click on it and select format but dont actually format the sd, just look at the reported capacity and make a note of the size (e.g 249gb) and compare it with the table below
if your SD is already in the Vita then look in system information at its Capacity (NOT free space) and compare it with the list below
a genuine 128gb card will show no more than 119gb
a genuine 256gb card will show no more than 239gb
a genuine 512gb card will show no more than 478gb
a genuine 1tb card will show no more than 955gb