What has improved in music and sound that I would need to.... reverse, so to speak, to get that 1950's/1940's sound in my vocals?

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The idea that there have been no technological improvements is pure nostalgic nonsense. Nostalgia often normalizes technical limitations as ideals rather than the barriers they were. In film, 24 frames per second was not some magical ideal but a practical standard we are now used to. Most people the. And now would certainly choose stereo over mono.

With that said, the ideal way to approximate that sound would be via literal usage of vintage equipment. Otherwise you are looking at the poor facsimile of plugins emulations

Find old gear or what sounds vintage. I think about harmonica microphones. Jack White and Primus are using them for distorted vocals.
I would also searxh for Izotope vinyl plugin. It's free and can give you some "old" sound.

While using old 50's gear would give you the best results, it would require remortgaging your home to buy it or finding someone else who has it and probably charges a fortune to use it. So it isn't practical. Let's work with what you have accessible to you as I suspect that's probably what you want.
 
I have found what works best for any vintage sound is not to use old or crummy stuff you have lying about unless it really is vintage. Instead, use the best gear you have and record a good, high quality, modern sound to start off with. That might sound counterintuitive, but what that will let you do is accentuate the qualities you are looking for and reduce the ones you aren't. Obvious plugins to use would include a tape emulator, consol emulator and valve saturation as well as EQ of course. Recording in the 50's used a much more simple set of gear. Usually just one or maybe two mics with the mic position in the room being used to create the mix. There were no big consoles with loads of channels and usually only 1 or 2 tracks on the tape. They had pretty basic effects and signal processing too. So avoid modern compressors and make the most of the tape, console and valve saturation for compression and subtle distortion. Don't be afraid to run it through multiple tape sims either with subtle wow and flutter and other tape characteristics such as hiss. A bit of degraded analogue slap-back delay should finish it off. Use a good reproduction of an original 50's recording for reference and you should be able to get pretty close.

As well as recording techniques, one take no autotune!, there will be some stylistic elements to capturing the sound that you are looking for i.e. the way that some words are phrased...

I do traditional (old) country mostly.  An SM57 or 58, no EQ on the singers mic and no reverb seems to get a vintage sound.  I don't recall much reverb at all on early stuff.  I was the "juke box man" for a company in Harrisburg, Pa in mid 60's.  I did all the record purchasing and programming of the companies juke boxes.  Very little to no effects on most recordings, whether jazz or pop or country.  Some reverb on some of the early rock. 
 
  

They used ribbon mics and that U47.
Ribbons don't cost and arm and a leg anymore but due to very feeble output, require a preamp with lots of QUIET gain. That's expensive. A ribbon does not have a lot of top end. The U47 is like butter but costs just a wee bit more. ;-).

This is about the closest thing to a 1950s studio that uses 1950s techniques there is. Well worth watching the video.

https://www.soundonsound....es/recording-50s-style

With the exception of the room, microphones and “mixing by microphone positioning” most of it can be emulated in a DAW. Pultec style equalisers, LA-2A compressors, tape emulation and working in mono can get you a long way towards a 50s sound.

Waves Abbey Road plugins, especially the REDD desk, J37 tape emulation and plate reverb are intended to emulate the technology EMI were using back then and sound pretty good.

Just to add -

Many 50s studios couldn’t handle frequencies above around 12KHz well, if they could record it at all. Low bass was also something they had trouble with, and mastering for vinyl/78rpm usually meant slicing off, or at least reducing, a lot of power in the bass as most 1950s playback systems couldn’t handle it.