"What is the learning curve for a vertical mill?"
Kinda depends on how "into" it you get - if you just want a "drill press on steroids", then not much. You'd need a normal drill chuck that's compatible with your mill (R8 taper is common on a lot of mills) - after that, a vise and some T nuts to mount the vise on the table. Most small to medium sized mills have the same basic functions as a drill press, just beefier. To drill a hole, you put a bit in the chuck, tighten it (using ALL THREE holes; this is METAL working) then position the work under the bit, turn the mill on (set speed FIRST), grab the quill handle and drill.
IF you actually think you'll want to use it as a MILL (flattening, cutting grooves, chamfers, etc), the cost AND the learning curve gets steeper - first would be the cost of the mill itself. That type operations cause quite a bit of side-loading, so a "square column" mill becomes desirable - the "round column" versions are lower cost, but it's harder to locate the cutter EXACTLY back where you had it before, and harder to keep the mill head from wanting to twist under load.
Then there's the cost of "add-ons" so you can actually DO something

- it's pretty easy to end up with several HUNDRED in "accessories", such as $50 clamp kits, $150-$600 vises, $60-$200 tilt tables, $150-$500 rotary tables, collet sets, boring bars, on and on...
I have this mill
Heavy-Duty Mill/Drill with Stand and Power Feed | Grizzly Industrial
And probably over HALF that much in add-ons, maybe more
But when the hydraulic pump went out on my old Case backhoe and the "new" one's mounting holes didn't quite line up, and the vendor claimed nobody else had that problem, a center-cutting end mill fixed it in maybe 15 minutes including setup -
When I needed 2-1/2" CHAMFERED holes in the QA project -

Multiple operations, multiple vises -

Needed "dog point" set screws in 5/8", couldn't find - add a small (3") lathe chuck with R8 shank and a tool holder (don't have a lathe yet) -


There's more, but by now I've either scared you away or made you WANT one real bad

But hopefully this "micro-course" gives you more of an idea... Steve