I saw a question about ferries around Borneo and I'm in a similar position. I'd like to make my way through Vietnam by whatever transportation means available, and then visit a friend in Brunei.
Are there ferries from Vietnam to Borneo?
No, there isn't. More generally, as far as I'm aware there are no scheduled international ferry services from Vietnam to anywhere.
The easiest (but not the cheapest) option for getting to Brunei is the direct flight from HCMC to Bandar Seri Begawan on Royal Brunei. Alternatively, and much more cheaply, you can fly to any number of towns in Borneo on Air Asia by connecting through Kuala Lumpur.