Can anyone suggest the most straightforward way to do this?  For example, replacing the Riftborn's ships with Vodyani?

I've made a few attempts too convoluted to describe fully here, but they involved modifying technologies, HullDefinitions[TimeLords].xml, ShipDesignDefinitions[TimeLords].xml, and others.  I got to the point where Vodyani hulls would show up in the Riftborn ship design screen, but with Riftborn names and in the main Galaxy screen the ship icons were still Riftborn.  I also couldn't build any of the "default" designs, only the ones I designed myself.

Or more generally, which XML files are involved in:

1) Making hulls available in the ship design screen with their proper names?

2) Making ships/hull types available to build in the planet construction screen?

Any help would be appreciated...

Actually, what would probably be the easiest is using the Custom Faction UI already in the game and change the Visual Style. 

Otherwise, yes, it's a not a trivial task. 

Ah, I was afraid of that.

Is it possible to give the Vodyani the base Riftborn traits that are attached to their visual affinity (no growth from food, Machine Embodiment)?  I looked around a bit, but I couldn't easily identify the Machine Embodiment ability...hopefully it's not hardcoded somewhere.

FYI, Machine Embodiment is located at the bottom of ConstructibleElement_Industry[StarSystemImprovement].xml and is called "StarSystemImprovementCreateMajorPopulation."  There's also the industry-to-manpower build in there, called "StarSystemImprovementIndustryToManpower."  They have faction prerequisites attached, so I had to make copies of them and change the prereqs to Vampirilis.  That required a change to GuiElements[StarSystemImprovement].xml in order to get the icons/text to show up properly.

There are also no-growth and no-food traits (among others) in PopulationModifiersTraits.xml and SimulationDescriptors[Population].xml that I basically just copied from TimeLords to Vampirilis.  It seems to be working.

I'm presently working on a quick mod to add Vodyani ships to any faction. I want to play a few rounds with it before uploading.

That would be great. Hope you keep us posted here!

That was fast!  I'll check it out ASAP.  Thanks for this!!

You're entirely welcome! Please let me know what you think.