Some thoughts -
Your vacuum pump on the truck should make about 20 hg of vacuum. Your VRV just lowers the vacuum going to the modulator (7-9hg as I recall.) at a specific point of travel of the IP throttle shaft. So it does not make vacuum but rather lowers it as it goes to the modulator on your transmission.
From the factory both hoses wrapped around the left side of the air cleaner, your top hose goes to the right. Make sure the VRV in hose goes to the vacuum manifold spitter on the firewall. The out goes to the modulator on the transmission.
1) Your VRV is maxed out towards the radiator, that as I recall is for a early shift like you mention. Loosen the adjusting screws and put it in the middle of its travel allotment, tighten, and drive again. Tell us if it improves. If not move as far as it will travel towards the front window. Tell us if things improved.
2) Ford says you can’t service the VRV if out of spec. (Can’t move it enough on the IP to or from to get a good shift point.). That is not true, you can carefully take off the plastic cover on the top that covers the top and the vacuum ports. Pull straight up lightly and it snaps off. Now there is a nut under the head of the VRV valve. Only the nut turns, the head and base do not, but it moves the head up and down. Be careful not to break your VRV. Now turn the nut one turn and test drive. If things get worse go two turns the opposite direction. Things get better turn 1/4 turn and test drive again. Continue to turn 1/4 turn in the direction of improvement till it shifts as you like.
Also check your vacuum line from the VRV to the modulator to make sure the rubber hoses at top and bottom are not cracked. You can also pull the vacuum line off the modulator and check for ATF. If so replace the modulator. But is far more common the vacuum pump is not working or the VRV is going/gone bad.
Also read this thread on the same topic. Discusses options for a new VRV. Also the Ford instructions on how to set a VRV.
So I'm new to the diesel specific C6 transmission, and I just realized that the one on my new truck has a cracked tube which is making it shift poorly. I did a search and it appears that as of 2019, these things were pretty scarce. I'd really like to put a factory replacement part on it, but I...
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