I think that's too much to think about while doing the skill.
Stand tall on take off, try to lift the back of your head upward as you take off, grab your shins until it's time to open up for the landing; the rest will fall into place on its own, and you shouldn't need to think about the finer details mid-skill. I'd go knees apart until you're comfortable with the landing, then start working it with knees together.
Depending on how your tuck syncs up with your rotational speed, you may be coincidence end up keeping the thighs aligned vertically as you pull into your tuck, or you may not; it's not really important enough to be worth focusing on.
A front tuck isn't a super-technical skill; it's more just a question of having enough power on takeoff and having the necessary core strength to pull into a tight tuck. The only aspect of it where technical detail is important is the takeoff, but even on that you've got a fair amount of leeway.