Darn juveniles! It is hard to tell. I have both vlamingi and unicornis.. I've had them both since they were really young and I can't remember what they each looked like.
Here's what I know. In the 2nd photo, I can see what looks like a blue line behind/under the eye. If you see that, it's vlamingi. Unicornis doesn't have any of that.
Unicornis develops colouration around the peduncal spine area very early. (blue, obviously) Vlamingi doesn't show any colouration in this area for a long time and even then it isn't blue.
You'll know pretty quick either way, because a vlamingi will show dramatic growth, while the unicornis is a much slower grower.
Both are commonly sold as "unicorn Tangs". Sadly, vlamingi seems to be much more commonly sold and it is rarely appropriate for most tanks.
