If I wear a tampon, and there is no blood on my underwear does it confirm the bleeding is from the vagina?
I would assume that, yes.
I also assume you’re talking about specifically when you are on your period? If so, then it is totally normal for the blood from your vagina to travel forward toward your urethra, staining your pad in front instead of directly underneath you. At least that has been normal for me. It just seems like the labia hold together (especially if you’re sitting) and the liquid has to find some way out. And it won’t really travel backwards. I’ll get air or water trapped there sometimes too (various reasons for that), and it will come forward too. It’s such an odd feeling. Does that answer your question?
If you are not on your period and there is blood coming from your urethra, then yea you should definitely seek medical attention.