I like the use of ligatures to write <= as ≤

I disagree; if you want ≤, use ≤.

Oddly, Iosveka currently doesn't provide (for instance) ⇒ or ⟹, according to the specimen sheet, even though it contains ligature-derived glyphs that look just like ⇒ and ⟹.

I want ≤ but I don't want syntax errors in my code.

Well, yes. For some languages we'll need to wait for standards committees, but there are a number of popular single-implementation languages that really have no excuse.

How do you get the ligatures to work with vim? I've been trying to wrangle PragmataPro and gvim for a while but haven't managed yet.

If you use vim in the terminal, ligature support just depends on the terminal emulator. In Linux, qterminal and konsole work.

Which color scheme is that, if you don't mind? I always keep an eye open for pleasing colors :)

Of course not!

The color scheme [0] is my fork of tango-plus [1] with a few small changes.

[0]: https://github.com/aminb/tango-plus-theme

[1]: https://github.com/tmalsburg/tango-plus-theme