Congrats on finishing Syntorial!
1. Typically triangle or sine. Sine may be a better choice for a more natural sounding vibrato as the Triangle has a slightly more abrupt U-turn at the top and bottom.
2. I don’t think it’s a simple as choosing the right waveform. Rather it’s your whole sound that matters. I think any raw waveform will sound kind of corny with vibrato. So it’s more about what you do with that waveform that matters. Like…
3. Portamento. That’s a good idea. Adding additional bends between notes could make it more natural.
4. It’s kind of redundant. I never use it. I remember mentioning it somewhere in the Serum Lesson Pack but can’t off the top of my head think of the use I demonstrated. If you don’t need it, don’t use it.
Can you post an example of what you’ve made so far? It may not be your vibrato settings, but the tone you’re working with. Like you said, those two examples are actual vocals, versus your synth.