There's two ways to look at this.
1. The US will be involved in the fighting actively the entire time. Say it continues until 1985.
A LOT of things that were developed in the late 70's and early 80's will not get developed or will be significantly later) due to fiscal restraints. If you're buying huge quantities of the stuff you're using right now in the war, that's what you continue using. A large chunk of the money you'd spend on R&D goes to operational expenses (paying for bombs, bullets, food, men and services). Yes, a lot of the weapons and systems that were deployed in the late 70's and early 80's had starting points in the 50's or 60's, but the money just won't be there.
Vietnam is NOT like WWII for the US. In WWII, the US happened to get lucky in that the generation of "stuff" they were tooling up to build or were building on December 7, 1941 was stuff they could fight the war with. It may have cost more in blood than the perfect version of it, but it worked. The stuff under development continued to be developed, because the US was both isolated from enemy action and had the economy to support both a mass army and the ability to build all the stuff while simultaneously paying for research. Case in point, the Manhattan project. 2 billion in 1945 dollars. Second case in point, aircraft types.
2. The US is NOT involved in the fighting and sloughs the burden of the pew pew part of the war onto the ARVN. The US provides a type of lend lease to RVN, which lowers operational costs significantly, which allows perhaps development but at a lesser rate.
A few things to also consider in the event of the US participating fully in combat -
The M-60 tank was difficult at best to run around the jungles and bridges of Vietnam, as I understand it. The M-48 weighed significantly less and was smaller and thus able to go places the larger tank couldn't. The M-1 is heavier, and I think, but am not certain, that it has higher ground pressure on its treads, meaning its more likely to sink in mud. I'm not sure about the Bradley, but its a pretty big chunk of equipment itself. Might have similar problems.
B-1 - I assume the point of using the B-1 would be to reduce losses from SAMs and fighters. Unless a much larger number were produced, I don't see them being used in a tactical situation. They were pretty much going to replace the B-52 in the nuclear strike role in SIOP, which would permit more B-52 strikes, I suppose.
B-2 - Probably not developed in this scenario due to cost.
A-10 - Tough plane, great gun, but if you can't see it, you can't really shoot it up. Ho chi min trail or in the thick jungles. There may have been a single squadron deployed for live fire testing, but the vast majority of those produced would go to Europe.
F-14 - This would have seen much enlarged production as it came into squadron service in 1974. It was not a ground attack platform as it was optimized for destroying massed soviet strategic bombers, but during a continued Vietnam war, it would have permitted the F-4 squadrons to focus on fighting the war, and the F-14 squadrons would have served with the carriers in the Med and the north Atlantic.
F-15 - Also air to air, so would have equipped squadrons as historically, but the planes replaced would have been sent to Vietnam, rather than the air national guard.
The Navy would likely have seen a reduction in everything but carrier construction and guided missile destroyer construction. I would assume that the continuation of a hot war for the US would involve a breakdown in negotiations, so you would likely see all the Iowa class ships reactivated and taking turns on the gun line. Perhaps even get all four of them together to bombard Haiphong.
I don't know enough about small arms to feel I should offer an opinion.
Belushi TD
alternate-timelines.com
