Hello C.C. -- Your 20 dong coin from Vietnam is modern, non-precious, and was minted in somewhat high numbers. However, it is exotic, cool looking, and slightly collectible in superb, fully uncirculated condition.
Since you report 'little or no wear' you probably have an almost uncirculated (AU) coin. Fully uncirculated coins must be in a holder of some type.
Here are some approximate catalog values for these coins:
worn: less than $1 US dollar
average circulated: $2
well preserved: $4
fully uncirculated: $8
These coins were made with two different inscriptions on the side with the rice farmer.
Coins with 'CHIEN-TICH THE-GIOI CHONG NAM DOI' are worth about a dollar more than coins with 'NGAN-HANG QUOC-GIA VIET-NAM'
Use the Important Terminology link on the left to understand what 'catalog' means.
Any coin that has been damaged, such as a coin that is bent, scratched or holed will be essentially worthless.