Single entry means one entry. Leave the country and the visa is done, even with 80 days of validity left. Multiple entry means come and go as you please until it expires.
Vietnam prices the choice directly: $25 single, $50 multiple, both up to 90 days. Buy multiple if your route crosses borders, like Thailand to Cambodia to Vietnam and back. Buy single if you fly in and fly out.
Turkey gives Americans a multiple-entry e-visa by default, 90 days within 180. Indians get single entry, 30 days, and only with a qualifying US, UK, Schengen, or Ireland visa in hand.
The wrong choice costs real money. A second single-entry visa mid-trip means a second fee and a second 3 to 5 day wait, possibly from a hotel lobby. When in doubt, the $25 upgrade to multiple entry is the cheapest insurance in travel. Sources: U.S. State Department VisaPack flags the right entry type for your actual itinerary during its $9.99 one-time per destination document check.