I was wondering if any company ever made a vacuum advance tach drive distributor for an "la" (318,340,360) motor?

Nope. Not for small blocks at least.

I have/had a few tach drives (Accel, MP) none of 'em had vacuum. I guess the reasoning is that if you want/need a mechanical distributor advance mechanism, you're probably wanting a very quick advance curve (or locked out timing) and are not terribly interested in whatever economy a vacuum advance would provide. Since race cars spend most of their time in a narrow rpm band (above 3,000 rpm lets say) there's little part throttle operation where vacuum advance would be of any benefit.

Corvettes and I think older T-birds had them so I could see where your idea came from. I've thought about it myself. No such thing as far as I know though.

You might see if a std vac adv 2 plate setup with the pivot would fit that dist & cut a window for the can. It may or may not be easily doable. Comparing them side by side would answer that Q pretty quick

I posted something a little while back asking about parts interchange between the regular vacuum advance distributors and the MP tach drive. They're made by different companies. You might be able to swap the entire mechanism over from the vacuum unit but I never tried it.

I've also wondered if a mechanical tach can be converted to electronic. You'd basically keep the tach face, needle and housing and replace the internal components with electronics.

There's a guy on Ebay (gaugeguy maybe?) that takes your older electric tach or gauges and updates them with new circuit boards so you don't need a tach adapter. Maybe he could tackle that job. Just a thought.

Not sure if you want the cable to be visible for the 'look'.