I have been doing extensive testing on my fighter since M16 preview.
In my testing of different combinations, I have made Vanguard builds for every situation (Group, Solo, 18k counterstat and 30k counterstat content). While I find the vanguard to be in a very well balanced state, and a very good option to the other 2 tanks, it can feel a bit boring in high level groups as partymembers tend to be able to survive without much help from the tank. In pre-mod 16 content, the solution in this situation would be to move on to the Conqueror path and be a DPS GF.
Finding a build for Dreadnaught that can be viable for anything other than solo campaign play is however impossible, as its Vengeance mechanic shines when taking damage/blocking incoming damage. The TAB-mechanic forces us to stop playing to charge up, resulting in other DPS in the group to finish the fight before we are done squatting.
In my opinion, the best way to fix this issue is to have the Fighter be the 2nd Support/Support class in the game. Having both paths que as tank roles, will make sure the players that are done protecting their party with the Vanguard options, can move on to a more "rewarding" playstyle as a hybrid DPS/TANK. This option would also make all the mechanics of the Dreadnaught shine in group content, as the vengeance meter would be filled when blocking/taking damage that the tank role usually would recieve. It would be an easy fix for the devs, as there is no need for major skill/feat reworks (a few feats would need adjustment, as they will become redundant), and the only downside I can see is that the Dreadnaught would become a bit over the top in PVP content if it got the +40% HP from tank role bonus...