I don't think they are similiar to Vanguard. Playstyle is totally different (the most melee focused faction vs the most ranged focused) same with lore, estetics etc.
From a playstyle they are different. But otherwise there are huge overlaps. Both are remnants of the star union that want to reestablish it in some form, both are "pure" humans without huge divergents, both value militarism and heroism etc.
The major difference is the use of psychohistory by the oathbound, and thar their tech looks shinier. Otherwise they are lacking unique substance.
As if the sharkan would only have their infiltration abilities as a unique fetaure but were otherwise just regular humans, not even a subspecies like amazons or dvar are.
At least these are my worries now. I hope the dlc will convince me otherwise.
Edit: A version of oathbound I would have less scepticsm about could be the following:
Instead of being humans the oathbound could once have been administrative A.I.s independent of CORE. As such they still analysed data of hundreths of Planets and the social developments of thousands of years. They then want to use this data to recreate a new society, under the assumption they know better what to do due to their data sets. Making it a zeroth law rebellion (A robot rebellion with benefical intend by the robots)
But instead of total control ala CORE they manipulate you only so slightly, that you follow their plans without realizing. And if you diverge, they form corrective measures.
For this purpose they created robots and mechs in human design that shall emulate paladins, angelic figures and gods. And they appeal to hope as a primary human driver as it is a very strong human driving force.
This way they could keep all the gameplay and aestethics, but were a far more interesting and unique faction. At least in my opinion.
