What is the real difference between thai and Vietnamese cuisine? Since you use pad thai, are these cuisines very similar and only different very slightly?

They are fairly different, but it depends on the dish I'd say. In general they both can have a nuanced sweet flavor, but I'd say the similarities stop there.

Vastly different. I find thai food to be more sweet and spicy where as vietnamese food more sour and spicey. They both use chile's and other similar ingredients but the flavor, technique and combination of ingredients differentiation the two countries just like Persian food and Armenian food, not the same. I hope this helps.

No I think they are different just like Cubans and Mexican might use rice and beans and other ingredients but in different ways. Different seasoning and form of cooking. I like both and I do favor vienemese though