Riding solo: not a problem. Most hotel/guesthouse operators speak passable English, and pointing and gesturing will generally be sufficient for food, petrol, etc. Language can be a problem when your shitbox bike dies and a mechanic is required. As a general caution, the entire population will try its best to, at every possible interaction, take advantage of you, so inability to speak the language will leave you vulnerable in this respect.
Pavement: coastal roads are almost invariably paved and in fair condition. There are sections under construction, but it's not like you'll encounter prolonged sections that simply are not, and never have been, paved. As far as I know, there are routes along the HCM Trail (i.e. the network of roads through the mountains along the western border) that are entirely paved, though dirt can be found if sought.
Traffic: no worse than any other developing-world country. All that is required - even on Highway 1 - is your attention.
Accommodation: will be available without reservations in every major tourist destination, and should be available in any regional centre. If you plan to ride to remote areas (e.g. loops around Sapa, or into the mountains along the border with Cambodia/Laos), check in advance.
Water crossings: no
Bike: Honda Wins and Bonuses (WH125) are the most common. They're fucking horrid bikes, in every respect, but mechanics across the country will be able to repair them if anything goes wrong. Trying to find anything better (XR250s, DRZs, etc) is probably not worth the expense and bother. Cuong and Flamingo Travel are the two best agencies in Hanoi. A bike from either of them might kinda, sorta, maybe be reliable. A bike from anywhere else will probably require an engine rebuild every second day.
Navigation: take a GPS. The paper map that is often recommended (published by a government agency) is of some assistance, but the categorisation of roads is almost arbitrary and not a reliable guide (e.g. one that appears to be a major road might in reality be little more than a country lane), and signs are largely absent or misleading.
Bribes: laugh, play it cool, and ride away; you're twice their size.