Okay, there's a LOT of misunderstandings of how this is working here. It's really simple, but it's being needlessly overcomplicated.
You have Temp HP's that are granted from various sources, one of those being Battlerager Vigor. Temp HP's do not stack. So if you have 4 temp HP and you're hit by a melee attack for 2 damage, and you would gain 4 Temp HP from BV (18 CON) then you would have a total of 4 Temp HP since they don't stack.
The only new thing here is that Invigorating powers grant Temp HP that stacks with any Temp HP you already have. So assuming you have 4 Temp HP, and then hit with an Invigorating power, you gain 4 Temp HP that stack with the Temp HP you already have. So now you have 8 Temp HP. Not 4 of one kind and 4 of another, it's just 8 Temp HP, and they follow the normal rules of no stacking except when an Invigorating power is used.
So if you have 8 Temp HP from any source and then you get hit for 2 damage, you will have 6 Temp HP. You will then gain no Temp HP because you already have more temp HP than you'd be gaining. If you then attack with an Invigorating power, it will grant 4 Temp HP that will stack with the Temp HP that you already have, thus giving you 10 Temp HP.
If you want, here's a crappy analogy to explain how the Temp HP's interact. Let's say you have two friends, Jack (Normal Temp HP) and Diane (Invigorating Temp HP). Jack and Diane each give you money periodically. When Jack decides to give you money he picks an amount and then only gives you enough money to get up to that amount. If he wants to give you $4 and you have no money, then he gives you $4. If you already have $2, then he'll only give you $2 to get you up to $4. If you have $6, he calls you a rich ******* and doesn't give you anything.
Diane, on the other hand, is very generous. She always gives you $4 no matter how much money you have. If you have $0, $2, $4 or $6, it doesn't matter, she'll give you an extra $4. Understand?