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
When you say ‘residential house registration’ do you mean the blue house book? So that would be a copy of that, signed by the registered person on the book, do you also need a copy of their ID card?
Or do you mean a residence certificate from immigration?