It’s a multiple entry visa. You may come and go as you please during the validity of the visa. Each entry will get you a 180 day stamp. You do not have to stay for the whole 180 days.
For a recently submitted application via HCMC, address in Thailand was entered in the previous page, no proof of rental contract was required.
For a recently successful application via the London embassy, an address in Thailand was required, but no proof of any kind of booking or contract was required.
Anonymous participant there is a list when you enter the visa website and choose Vietnam as your location. It lists the towns and cities covered by Hanoi and HCMC