You don't need to do US bookkeeping then you don't need to do this! If you do (you have a permanent establishment in the US) then you need to do bookkeeping, its quite expensive since salary of accountants in the us are higher than other countries especially countries like Vietnam. You can try hiring indians on fiverr or that but I wouldn't recommend.
No, don't need any forms.
1120 is only for corporations. foreign owned Wyoming/Delaware LLC without PE in the USA then only cost is registered agent + franchise tax. FBAR they don't care about if you are not liable for any taxes in the US, I wouldn't bother