i think I've heard that you can use Vocalistener on UTAU, but some people consider it cheating and think that it makes it so that you don't have to put any effort into tuning at all
There's no "instantly more realistic" plugin. All plugins are simply utilities, and it's up to the user to make the most out of them. I personally find bizz's iroiro very useful, as it combines multiple functions into one. You may find some of these plugins useful for tuning: http://z-server.game.coocan.jp/utau/utautop.html
i think I've heard that you can use Vocalistener on UTAU, but some people consider it cheating and think that it makes it so that you don't have to put any effort into tuning at all
you cannot use vocalistener on utau, it for for vocaloid only but there is a similar plugin called utalis but the results have to be edited ALOT to get somewhat decent results
In general, to make an UTAU "realistic" it depends on the recording method, oto and most importantly tuning. Just because an UTAU is like 7 Pitch VCV, it won't necessarily sound realistic if there is no tuning; which is mostly pitchbends. Flags can also make a difference, but the most important aspect is using pitchbends. I wrote a tutorial on how I use pitchbends: http://utaforum.net/resources/sors-tuning-tutorial.347/