I see we are back to wander the light fantastic again here with various "options". Unfortunately lots conflated. Can't say that I'm optimistic about it, but here goes ...
Clongton's "double down" on DIV/RS68 does address an entirely domestic supply LV that has heritage, does the job, and is in production / has facilities. It's also the case that SLS SSME / RS-25 follow on share development with RS68, so its not a totally impossible idea. And there are some who might like another crack at fixing DIV's manifest "sins". Really what he's suggesting is "finish the hydrolox game that was selected back in Nixon's time" as follow on to Saturn V ... because you've got it, you use it (til 2022+), and you'll use it again (SLS).
Trying my best here to make that indigenous approach work, lets say you take the parts of AJR/ULA and make a hydrolox EELV that meets the next 7 years of EELV til Vulcan.
So instead of AR-1, lets say you were to have a cost reduced and higher volume RS68, with a refined Delta IV to match, and you finish/refine pad facilities. It's true that from a financial/skills/design leverage position, you'll get the most per buck of new money in terms of improvement, and ... there are long neglected parts of Delta IV stages/GSE/pad/HIF that could be improved, and common US makes sense too. (BTW, AR has no incentive for cheap RS68 ATM, although if SLS was in operation, the engine contracts would, like with Shuttle, theoretically bring down costs.)
So ULA knows this most of all, why are they uninterested? Remember that $0.5B+ RS68 that under performed, the stage/pad/flow economics that didn't work out? Now, all the investments to "improve" that you didn't do, because you got a better "return" on advancing Atlas, get done. And ... you abandon them in 7+ years when you don't fly DIV anymore. And ... you don't get the cost savings for reducing down to two pads and few assembly lines/steps ... which are considerable. Not to mention not having to wait for SLS to happen for cost sharing ...
Clongton reminds us that Vulcan/"FATlas" is just a variation of Atlas Phase II/III, and that if one EELV can be used to advantage, so can the other.
Could you make this work? Yes. Could you get a cheaper hydrolox LV? Absolutely yes. Would it be financially wise? Almost certainly not. Unless you find someone brilliant who can reassemble these parts into a more effective launch service provider.
Which brings up an odd thought ... offer the AR "team", who wanted to acquire Atlas, Delta instead ...