Apologies for my slow reply, Sandy! The best time depends… I like winter, though it can be a little misty and cold in the north. Spring (e.g. April) is probably your best bet for nice mild weather in the north and not-too-hot weather in the south.

To be very honest I agree with your friends. I’m not a huge fan of Vietnamese beaches… they’re fine, but they don’t live up to any kind of idyllic tropical beach image. The snorkelling is often really bad both in terms of visibility and the number of fishes. Phu Quoc has nice beaches though. I’d suggest a resort there, or perhaps one of the Koh Chang islands in Thailand. They are developed, but aren’t quite as busy as other Thai islands. Koh Kood might be what you’re looking for. It wasn’t quite my jam as I was travelling as a backpacker at the time and not looking for resorts, but the beaches are absolutely gorgeous and the snorkelling was fantastic.