Ray Larabie

The fact that fonts in previous centuries had little to no kerning seems irrelevant to how fonts are made today. Those designers were working against certain technical limitations. Technical limitations create style. We're working against a different set of limitations. Thousands of kerning pairs combined hundreds of kerning classes leaves us with almost almost unlimited possibilities.
Language support beyond your likely customer base is there to make localization easier for your customers. Multilingual apps, packaging and instruction manuals are the norm these days. Apps have fluid localization. They often start in one language, gain popularity and languages get added one-at-a-time.

They're important because it's the only way to type Spin̈al Tap properly.

Agree...with the 3rd sentence. If you keep second guessing yourself you'll spoil it. Commit and move on.

Leave the the f ligatures blank in monospaced fonts—they should produce a .notdef character if someone hard-codes them.

I used to try to get the Vietnamese capital accents to tuck under the hhea ascender/WinAscent but now I don't think it's attractive. A lot of typefaces, including system fonts have Vietnamese capital accents that go over the top. Some applications will cut the accents off but I don't think any legitimate design apps will. Anyone setting text will have to increase the leading but I'm sure that's something people setting Vietnamese always have to deal with.

You can find some Windows core fonts with enough headroom to allow Vietnamese stacks. That's one way to go but with consequences for other languages.