Yes a vertical labret is on the lip surface to the lip edge, a labret is on the lower lip below the lip edge so having the two should not be an issue.
No. Vertical labrets don't touch the inside of your mouth at all. The holes wouldn't match up.
Yes it's called a vertical labret
Lips are pierced with barbells all the time, generally bent barbells for vertical labret piercings.
a labret piercing
If it's a vertical Labret piercing then it's a curved barbell, if it's a standard lip piercing it can be a labret stud or a ring.
A vertical labret is through the bottom of your lip like a normal labret but instead of the other side being in your lip (touching teeth/on gum) it is through the middle of the bottom lip. so you can see both balls on the bar. See pic: http://www.urbandictionary.com/define.php?term=vertical+labret
Yes, guys can generally get away with most piercings.
Vertical labret, spider bites. I personally think that vertical labrets look better on girls.
For a short time after it's pierced, it might feel numb or tingly. It's not normal if you've had it pierced for quite awhile and it's still numb.
Typically, a tongue piercing may hurt more than a lip piercing, as the tongue is a sensitive and muscular area. However, pain perception can vary from person to person. Both piercings may cause discomfort during the procedure and while the area is healing.
Labret is french for lip