Hard to say given the little info from the OP. How many days, what bike, what month. For sure is avoid the TET Chinese New Years.
I took the whole winter to do Vietnam and China to Chengdu, 4,200 miles. Weeks in Saigon, Hanoi and Chengdu. From Saigon to Hanoi was Dec. 5 to 30, 2014. The ride to and around Vung Tau is GORGEOUS, but had not enough cash and this place had none so, I actually had to go back to HCM to get 16 million in my wallet. LOL. I then had a poor start because Malaria pills made me feel zonked until I stopped taking them. Had 3 vanxi rides and a 180 mile bus ride to Hoi An.
I really don't see the hype of this super crowded shopping bazaar with boat rides. I don't bar hop either. I did get there after dark at a hotel on the outskirts and it was spitting rain when I got on my bike and rode thru less than an hour. I just don't get the hate for DaNang, must have 8 miles of beaches and ocean waves and MUPs. Traffic is lighter compared to Hanoi, but is horrible to walk across the main drag with few lights. Checkout Two Wheel Cruise on YT for these two cities.
Halong Bay tours are fun, but you choke on diesel fumes that also choke the view. I did that one day bus tour from Hanoi in 2007. That year I just flew in and spent 5 days in Hanoi and 15 in Saigon, riding a cheap little girls bike I bought, and used my own tools I took. LOL.
So I hope you have 30 days, but I suspect you only have 14 or 21. If so don't bother with the muddy mountain torture test, it's just not worth it that I've seen on videos. Same slop as Laos. I HATE gravel anywhere.
I made sure to stop in big cities and big hotels, they were $40 to $60. If you are OK with $20 hole in the floor shower rooms, there's hundreds of them too. I'm a picky fast food eater, so NO pho soup and not much banh mi sub sandwiches. EVERBODY gushes over these. LOL. Had a mostly OK hotel breakfast and seldom found anything for lunch but fruit, Snickers bars and they only had orange juice among all the poison spiked drinks and pop. Yah chips are awful in VN. There are buffet places, just look for a line of trucks at noon. Even there I was lucky to find some bird meat, tomatoes and plain rice. Duck is rather awful IMO. What is good are the bakeries present in many small towns, buns maybe with eggs or weiners, little cakes smothered in icing.
For supper it was burgers at Lotteria and McD or the many VERY good KFCs, where I got 2 fries and 3 pieces to fatten up. I know I'm the only guy doing this, LOL. McD burgers actually pretty small and their Mc chicken has absolutely HORRIBLE hot spice same as many States, nothing like in Canada, but I haven't been there the last 2 years either. There's some soso Burger Kings. I hate pizzas. Street vendors will only give a speck of meat, but yah it's dirt cheap. Half the fast food was in the Big C grocer malls, now called Metro or something.
I ride freeways around Edmonton, so I also rode the coast and lots of Q1a. Get the hell over when a bus comes along, beep beep means there's a bus stop. Truck drivers waved at me, LOL.
You DON'T really need to wimp out of city traffic, especially with experience on the Toronto lanes. Expect salmon MCs, give them the gutter. Lots of times I helped out cars and trucks by blocking the MCs butting in.
The fort in Hue is a must see. I had time to see the zoo in HCM. Most guys see the boat caves tour in from Ninh Binh. I didn't mind some rides on side roads if they didn't end in dead ends. Lots east from Hanoi.
So have FUN riding up Hai Van pass, lucky if it's not raining.
Some pics >>>
My fave kind of breakfast for 3 weeks on Pham Ngo Lao, # 275 I think. Was about $5 with juice, didn't starve there for sure. My hotel was old and dumpy, but it had tubs and I had stayed there in better times, 2007. Nowhere is quiet in big cities. January is the smoky cane burn season. I didn't mind losing 10 lbs, mostly because of skipped lunches. I started every day with 3 bottles of water, now iced in Kleen canteens, and at least 2 OJ, leaving Hanoi I had 7. I bought 6 snickers bars where I found them. LOL.
Cakes >>>
Truck crash in front of my hotel in Vinh while I was sleeping, LOL. >> That'll buff right out.
My 120 lb Rohloff14 bike in beautiful Vung Tau >> Only flat was from a patch that came loose. Make sure you have a good air gauge, there was none for sale there. I sat on mine and broke it in Hoi An. Big oops. I also goofed not having an inches ruler for the chain. Another need, IMO is a bunch of shoe and 72" skate laces to tie stuff. None there either. I had to tie plastic bags on my shoes when it was raining and the shoulders were soupy mud from tractors I guess.
In 2019 I was riding my CCM 3 speed from Markham to the Toronto shore 4 days, easy.