TL;DR: every point in vigor is worth more than the one before it, up to 40 vigor, and it is almost always best to hit 40 vigor as soon as possible, on all builds.
There’s probably some folks playing Elden Ring now who either didn’t play it at launch/dropped out of it, or struggled through a first playthrough and never went tremendously deep on mechanics. This may be for you!
If you’ve ever complained about the difficulty in Elden Ring somewhere that someone can reply to you, “level vigor” is probably the second reply you got after “git gud”, but it helps to be able to visualize it.
If you’re familiar with dark souls games, or simply have an instinct for progression balance in RPGs, you’d probably expect that early points in vigor afford bigger bonuses to HP, and that those diminish as you increase it further- it worked that way in three previous souls games! It makes Sense! But for whatever reason in Elden Ring they made it so that until you reach 40 vigor each point actually gives you MORE HP than the one before it. This means that vigor is almost always the best way to spend a level up until you have 40.
It’s easy to see how big a trap this can be! If you play a caster or find a weapon with high requirements early that you really wanted to use, it’s very easy to invest in the relevant stats, especially if you see the hp difference between 11 and 12 vigor and go “eh that’s not THAT much”. But it means you end up with dramatically less health than a person who does invest in vigor, and unfortunately with the way enemy damage grows, you’re seriously going to need it by the time you reach level 60 or so. To further tip the balance, weapon/spell scaling doesn’t really pick up until higher upgrade levels anyway (~ +12 normal, +5 somber), so any points in damage stats beyond meeting weapon/spell requirements are also less useful in the early parts of the game. Apart meeting requirements for what you want to use, there really just isn’t a reason to invest in anything other than vigor until you have 40 of it.
If you’ve been struggling I hope that helps!