Looking to motorbike with some fellow travellers.

You should have no safety problems when riding during daylight hours. It is not advisable to ride after dark. Firstly, because of the danger of potholes in the road (even on otherwise good roads); and secondly, because highway robberies do occur at night, and if something happens you are a long way from help.

By connecting up stretches of coast road, it is possible to get most of the way from Vung Tau to Hue along the coast. Between Phan Rang and Nha Trang there are some particularly beautiful cliff roads overlooking the ocean.

Use online maps, including Google maps, plus the blue Vietnam road atlas [Tap ban do giao thong duong bo Vietnam to help you. Neither have ALL the information you need, so ask at your hotel as well.
If you get to Khanh Hoa, I can show you some interesting routes. Send me a PM.