Mercenary Kings Reloaded [playstation.com]
was released today for the Vita (as well as the Switch and other systems), If you got the PS4 version (include via PS Plus) it should show up as free shortly.
UPDATE: The PS4 listing has been updated, but first add that (shows as a free purchase if you already owned the game on PS4, even via PS+) then purchase (for free) the Vita version
This is according to Tribute games, even though they have acknowledged a current issue here [twitter.com]
Note if it shows up for 19.99 and you own the PS4 version, the issue has not yet been corrected.