Oil used to fry food

You can be sure about that nobody, especially street stalls cook with peanut oil 
They just cook with vegetable oil which cost below 25000.000 VND per 1 liter.

In my Chinese family peanut oil is considered fancy and I know the prices of that were higher in US, so I'd guess not. But it'd be wise to learn your allergy in Vietnamese, I had to do so with medicine allergies.

They use mainly vegetable oil, but be aware of cross contamination.

You'd be surprised how many dishes contain peanuts as a garnish, or textural ingredient. Peanut is "Đậu phộng" in Vietnamese, pronounced approximately "dow fom", both syllables in a low flat tone, but ask a Vietnamese to teach you how to pronounce it. Oil is dầu (yow or zow, descending tone), but as others have said, street vendors are unlikely to use such an expensive oil. You'll probably only see it in the more expensive restaurants, where it's likely you can ask in English. No guarantees, though!

If your allergy is life-threatening, keep in mind that cross-contamination can occur if they use peanuts as a garnish in a dish you haven't even ordered.

Peanut oil is a type of vegetable oil, and can be, although very rarely is, included in blended vegetable oils. Peanuts are legumes, not nuts.

Most blended vegetable oils in Vietnam are predominantly soy bean oil.

So sorry for the delayed reply! Thank you very much for your responses! I really appreciate the guidance

I think there's no way to assure safe food without peanut oil. Even seller may don't know what oil they are using, especially street food. The safest way is to rub the food on sensitive skin area and wait for reaction before eating.
Regards!

With a peanut allergy you are seriously at risk with many foods here let alone deep frying. The best oil to use is coconut oil but nobody uses it because it costs a little bit more. Food wise you are now in the Wild Wild West! I like it but not good if you have an allergy. Keep your antidote with you at all times.

I truly fear for you as peanuts play such a big part in vietnamese cuisine