guide that walks you through how to start playing games on vita3k using the (decrypted) .zip install method. also shows you how to install a patch for use with vita3k using the diabolik lovers english patch as an example. another guide for playing and patching games for vita3k, using the (encrypted) .pkg method with the hakuoki ssl english translation as an example, can be found here.
download vita games straight from the playstation server. a bit more complicated to set up than than just downloading from a rom site but safer and has a built in unpackager. follow the 'configuration and usage' section, following the areas that say 'PSV'. you can also download .pkg files straight from their site under the browse section, which also has zRIF codes/work.bin files (vita3k will ask you for one of these when you install a game through the .pkg method).