Cinjodi

I'll be in Vietnam for a month and expect to use Grab quite a bit. I've downloaded the app to my phone but I haven't set up an account yet. When I travel I usually buy a local SIM card rather than buying an international plan with my US carrier. I've been holding back on setting up a Grab account while I'm still here in the US because.. wouldn't that be under my US phone number? I don't know what my VN phone number will be until I get the SIM. Do y'all usually change out your SIMs when you travel, and if so, how does that affect your Grab account? Am I correct to wait until I get to VN to set up the account after I have a phone number in hand? Looking to the future, what do I do when I travel again to VN and use a different SIM...? Is it easy enough to change the account or do I just ditch the original one and establish a new one?
Sorry to be so dense about this but I want to make sure I do it correctly.
TIA!