I was in Vietnam (Jan/Feb 2008) and I took with me Lonely Planet and Rough Guide, and the better of the two guides is, by a thin margin, Lonely Planet.
However, there is much to commend Rough Guide Vietnam. It is worth buying just for the section that covers the history, religion (13 pages) and the overall culture of Vietnam. Excellent.
This guide is well laid out; breaking Vietnam into eight sections. Each section has a page that points out the places not to be missed. Each of these sections opens with a very good history and explanation of the area. Rough Guide has an excellent selection of restaurant and eateries (better than Lonely Planet's) with enticing write-ups that tell you what to try, "Goi bo, a salad of banana flower, star fruit and pineapple" or "try stir-fried beef with lemongrass for starters, followed by fried scallops and then che baba - grandma's sweet coconut soup." This guide has a better selection of the discos and clubs than Lonely Planet and it has eight (8) pages of recommended books to consider - Super.
HOWEVER, Rough Guides hardly has any maps, and those that they have are not as good or easy to use as Lonely Planet. It has a cumbersome "price code" system for accommodations, i.e. 1= under to $10, 2= $10-15, 3= $15-30, etc., thus, you have to memorize what the numbers represent or flip back and forth to the legend. Whereby, Lonely Planet shows you the cost in dollars. What an idea! Duh. Only a few accommodations (very few) have webpages. In today's world accommodations webpages are a must. All savvy travelers today want to "see" what a hotel or hostel looks like. R.G. does not breakdown restaurants by cost (Expensive, Moderate, etc.) nor does it give you any prices ranges ($10-15 etc.). Not Good.
In short, the better of the two guides is Lonely Planet, especially if you are going to explore Vietnam and want to get away from the "tourist areas". However, if you are staying in HCMC, Hanoi or other major cities and want the best clubs and restaurants with good eatery descriptions then this is your guide. Strongly Recommended. 4 Stars.