I have a vein on the middle of my forehead. Does this mean I'm not a candidate for hairline lowering procedure?

Your forehead may appear bigger but it probably is an illusion when you wear glasses.  Or... your may have better eye sight to appreciate a bigger forehead?  Or ... as you noted it is just a perception when you wear small or tight glasses versus larger glasses.

In my opinion, to correct your hairline, a surgical approach is necessary. This can be hair restoration as well as forhead revision. As far as thickening the scalp hair, non surgical techniques exist. Finasteride is very successful at improving hair loss. Rogaine and Revivogen are also useful. I...

If you have post operative issues, it is important that you contact your doctor.  Shock hair loss is a complication after forehead reduction surgery.  Scalp irritation is a side effect of Rogaine.  You need to go to your doctor and address these issues. 

I commonly perform forehead reduction surgery (scalp advancement, hairline advancement) on African-American women and do not see issues with poor wound healing or keloid scarring. While this is certainly possible, it has not been an issue for my patients. Any noticeable scarring that might occur...

Hair loss after scalp surgery is possible, but rare. Permanent hair loss around the incision or other areas might be possible due to surgical technique issues. Temporary hair loss, known as shock loss, is also possible, but very rare. In these cases, the hair will fall out and then regrow over...

You need to have permission and consent from your parent or legal guardian before having surgery.  If you are mature enough to understand self esteem or happiness does not come from cosmetic surgery, your next obstacle would be getting your parent's permission.