Please check your bill again and post a new, separate question with its denomination. Vietnam does not use dollars as its currency.
It is going to vary in value depending on who sees it or is interested in it. It also depends on the condition of the bill. The more pristine it is the higher the value. Sounds like you have a rare bill on your hands, hang on to it and get it appraised, go to a coin shop or antique dealer. DO NOT GO TO A PAWN SHOP! You will get robbed!
You can hang onto it as a conversation piece but don't expect to get rich. Its value has increased less than 30% in over half a century.
No collection value but is has a scrap silver value of about 10 bucks.
Piasters were the primary South Vietnamese currency during the Vietnam War. And US MPC (Military Payment Certificates/and US Greenbacks were blackmarket at the time) were always preferred by the Vietnamese nationals when dealing with GIs. Any monies by any other name are stangers to the Vietnam War.
Depending on how "circulated" it is, it's worth between 25 cents and $3. Since it was circulating in the earlier years of the Vietnam war, though, it's definitely a cool keepsake item.
"Ngan Hang Quoc Gia Viet Nam" is Vietnamese for "National Bank of Viet Nam". The wording was used on the coinage (and, presumably, paper money) of the Republic of Viet Nam (South Viet Nam). As there was no South Vietnamese coin with a denomination of "1000", I presume that you are refering to a 1000 Dong bill from South Viet Nam. While I know enough to know that its value would depend at least somewhat on its condition, I do not know what the range would be, nor whether there were multiple issues of 1000 Dong bills.
McDonalds
depends how long you are able to hang on to them!
Son Doong Cave: is in Vietnam. At more than 200m high, 150m wide and 5km long, the Hang Son Doong cave in Vietnam is so big it has its own river, jungle and climate.
30000
Never heard of it.
Look at a map. It doesn't sound familar for RVN, try North Vietnam. Might even try North and South Korea; sounds abit Korean.