International roaming on Unlimited Premium; how to activate? Best network in Vietnam? Multi-network work abroad?

Hey all- I have a trip coming up, did some googling and searching the subreddit and didn't see conclusive answers to all my questions.

My situation:

• Unlimited Premium (Dark Star main line)
• 2 Multi-Network adds on for both Warp and Light Speed
• Going to be travelling internationally to Vietnam
• iPhone 17

My questions:

1. Can I roam on my current esims or do I need a specific esim for international use?
2. I know I need to flip Data Roaming ON in Carrier settings- do I need to do anything in the USMobile app to tell it what country I'll be in or is it just the carrier settings in iOS that I need to change?
3. Which of the 3 networks is best for roaming in Vietnam? Internationally in general?
4. Can I switch data between the 3 multi-network esims while abroad?

Thanks all!