Why are there dark-ish / black-ish veins under my tongue and is it serious? because looking it up saying it could be a vascular disease so.. i just want to know if its serious or not because i never really noticed it before..?

This is the color of normal veins against the backdrop of the pink tongue underside. Don't overthink it. If really concerned discuss it with your doctor at your next encounter.

What you described is consistent with the normal appearance of the underside of the tongue.

This does not sound abnormal, but it would be reasonable to show it to your doctor during your next visit. I would not worry about it unless you are having some symptoms or unless you notice a change. Good luck.

Probably normal. Just took you 19 years to notice. You should be seeing a Dentist every 6 months. Please know that your 5 minute Google search is no substitute for your Dentist's 10,000 hours of training and years of experience. To be sure, request an examination.