Not to be a downer, but most UTAUloids rarely get popular due to the large amount of them.
If you're aware of that, YAY LET'S GET AWAY FROM THIS NEGATIVE SPACE.
First things first, BE SOCIAL. Talking to other people and helping others out really strikes an epic chord in many people, and you will get more attention as so.
Second, remember the phrase "First impressions go a long way". If you release a new act of your UTAU, the first thing that people hear is what people will base the decision of "Should I listen to this UTAU sing *insert song name here*?" on. Making a bad first impression = very very few viewers. Making an amazing first impression = quite a bit more than making a bad one.
Third, don't be afraid to use other people's UTAUloids. In fact, that's kind of key to becoming known. Using other peoples UTAUloids for UST releases, choruses, and other videos make not only those UTAUs more well known, but your own stuff better known too. So, more viewers.
Fourth, MAKE ORIGINAL CONTENT. Be it PVs (which many Overseas users tend to be lazy with), songs (which very very few Overseas users tend to even know how to make), or MMD-PVs (which are made more often by Overseas users), anything that is original is a bonus point.
Fifth, it's good to have a surprise every once in a while. Do something that no one will expect, and it's noticed for the most part. Do something that you have announced and even given previews of, less people will take notice of the finished product.
Sixth, give constructive criticism, not destructive. Giving critique about other peoples stuff by telling them what to make better is MUCH better than saying "Vocal line is off time, I can't believe you couldn't tell." or "YOU FAIL" period. Critique should sound more like this- "Well, the vocals are off-time and a bit off-pitch, and maybe she/he's singing a bit to fast compared to the background, but other than that, it's good! I do like how *insert name of UTAU here* sounds singing this." Giving constructive criticism shows that you know how to use UTAU without making you look like a total jerk. Giving destructive criticism however, does not.
Finally, have FUN. This really doesn't help much in popularity, but that's kind of one of the many things users do when using UTAU- having fun! Not having fun means you most likely didn't put your all into making something sound great, and you don't feel that feeling of accomplishment when you're done. It's kind of a motivation!
I'm not sure if much of this helps... if anything, getting people to subscribe to you via other means also helps, because they'll watch your UTAU videos too, or at least they'll know you posted one. (But what the hell, I don't really get many views for my videos, the ones that do have a lot views have been up for a long time, so you're pretty much getting advice from a popularity newb XD )