Every Vietnamese restaurant I've eaten in (in the US) uses jalapeno peppers in their pho. I assume that's an American adaptation to the dish. Is that correct? If so, what kind of peppers are used in Vietnam?
I'd go with Bird's Eyes, or as they're sometimes called, 'finger' chillies.
I'd define it as skinny, thin skin, lots of seeds - finger tend to be larger, green; bird's eye smaller, red.
The trouble with chillies is that they can all look a bit alike & trying to buy one very specific type is really not easy. Is it a jalapeño, is it a fresno or a serrano… what's the real difference? Can you tell just by looking, or even tasting?
You can easily tell the difference between a scotch bonnet & a jalapeño, of course, not only do they look completely different, they taste & smell completely different, but a lot of the others come in much finer distinctions.
Finger chillies & bird's eyes are close to being the same thing. They're not identical, but to the consumer it's really tough to tell them apart. Supermarkets tend to call the larger ones finger & the smaller ones bird's eye - they taste the same. See https://www.sainsburysmagazine.co.uk/lifestyle/food/a-guide-to-chillies for how the supermarkets very, very broadly differentiate.
BTW, I can tell the difference between a jalapeño & a fresno… but not if the ones the supermarket is selling this week look like something half-way between the two.
Late thought - cayenne looks like a 'finger' variety too; All I can think is they call them finger if they're green & cayenne if they're red. They never really tell you exactly what they are.