Alex has already addressed the degree issue so I'll try my hand at some of the others. Re. Disclosure Scotland, it would be better for the disclosure to be as up-to-date as possible. Mine only took a few days to come through so there really is no point in applying for it now.
Thanks to a change in the law, business visas are issued for a maximum of 3 months. This means constant visa runs (usually by way of a cheap flight out of the country) BEFORE the expiry date. Consequently contracts are rarely issued for a year. It's more like 3-6 months, after which you will have to sign a new one. Note that this could involve a change in terms.
The school I worked for sent me a four page document on the strict dress code (no sandals/open-toed shoes for either men or women, no strappy tops, no visible tattoos/piercings, only one set of earrings in your ears, etc). In practice, this was not enforced, although had I have been teaching in-company or Business English, things may have been different. As long as your tattoo CAN be covered up, I wouldn't expect you to have a problem. That said, I taught in Hanoi where I imagine they are more used to Westerners.
In terms of being stared while out and about, as a foreigner in an Asian country it's par for the course, even in the big cities. This has little to do with your body art. Simply put, it's because you're a foreigner. Having typed this, I realise that I have assumed that you're Caucasian. If you're of Asian origin, you will blend in better, although finding work will be harder. If you're of Black origin, I'd respectfully suggest that Vietnam might not the best choice of location.
If you want to teach VYLs (4-6yrs), singing will almost certainly form part of your teaching. I just played the CD loud enough to drown out my tuneless offerings! I also included the odd song with some of my YL classes (usually only those up to about the age of 8) but if I sensed that the kids weren't into it, I happily skipped it. With older kids it's most unlikely that you would be singing but you may want to include a song for listening and/or teaching grammar purposes.
Hope that helps.