having left our copy of the Rough Guide to Vietnam in a taxi in Saigon during the first week of our trip, we had no option but to purchase a copy of Lonely Planet. We were very disappointed, if you stuck to the Lonely planet, you'd only see a quarter mile radius of where you land, and miss out on so much that vietnam offers. It felt as if the author lands at the local backpacker haunt, then does door to door reviews of the closest hotels, bars and restaurants. The reviews also seem to act as a reason for a price hike as prices don't reflect what the lonely planet suggests. Yes, there's inflation, but it was published only last year and the prices really did seem way off, even though we were traveling during the low season.
You'd be able to get by with the lonely planet, and if you're not an adventurous traveller and are happy to follow the crowd, then this would be a fine purchase for you, but the Rough guide seemed to provide so much more detail. The one positive in favour of the Lonely Planet over the Rough Guide, is the language section which provides Vietnamese phonetically which makes getting your tongue round the tonal language much easier.