Basic info on Vietnam

On question 2, Uber does not operate in Southeast Asia. Our equivalent is Grab, which operates in Vietnam. Or take on airport taxi.

It's definitely worth having cash for taxis (not all take card), Street food, small shops etc. Many smaller places don't take card, and Street food vendors certainly don't.

5) I used an Airalo data plan in your first 3 cities (in March) and it was fine - no problems at all. I was also in Hoi An. But I did not make calls.

2) I did the trip in reverse (hotel to airport in Hanoi) and my hotel arranged for a transfer. I am pretty sure any hotel can do this.

1) I used Lonely Planet and Rough Guides to help plan my trip.

2) I used a private driver to get to the airport (I departed from Hanoi). Sorry I can't remember how I booked it but it was relatively inexpensive as I recall.

3) You'll definitely need cash for food vendors. I was able to get dong in advance from my bank and also via ATMs in Vietnam.

4) I didn't do a food tour per se, but I did a tour with Hanoi Kids that was very enjoyable. I still dream about this noodle/bacon dish I ate that was incredible. They have changed their tour itineraries since I took it, but you'd need to have some free time to take their current food tour as it starts at 6 p.m.: https://hanoikids.org/ I also took a cooking class in Hoi An but unfortunately the tour operator is no longer in business. The food in that area is unique and delicious - if you can find a food tour there, that would be a good bet (though I see you're staying in Hue so that might be too hard to arrange in Hoi An)

5) I used my AT&T international plan and had decent reception nearly everywhere.

Agree with above, Lonely Planet!
Grab like Uber (You can pick Grab car or Grab bike).
Use ATMs or bring your currency to exchange for cash, VND easily at gold/ jewelry shop (Clean and big notes for better rates).
Regarding your itinerary, I would do this order Hanoi- Hue- Danang- Can Tho- Saigon (Fly Danang to Can Tho).
You can try XO food tour once in Saigon.

I bet they have a FAQ page on their website.

I'm always going to fall back on Fodors and Frommers, I'm too old to go the Rough Guide/Lonely Planet route.

Definitely have a good amount of cash in hand. My credit card was fraudulently used on my second day in Vietnam.