The maximum level in the game is 99, not 100. HP growth is set for each character. It is not affected by your Vitality stat in any way. Two characters follow a set pattern regarding HP, those being Yang and Cid. With Yang, his HP will cap at 6000, or when he reaches level 60 (so for example if you had 5678 HP for Yang at were at level 60 now, that would be your HP for the rest of the game). With Cid, he will consistently reach 9999 HP at level 99, usually beforehand.

If you're adamant about getting 9999 HP for every character then the only way to achieve this is via Silver/Golden apples. They appear throughout the game in chests, but can also be dropped by enemies. Your best bet for this is probably Lilith's on Mount Ordeals. For one thing they're reasonable common, and have the best chance to drop Silver Apples (Golden Toads for example have a chance to drop Golden Apples, but are super rare). Secondly they're very weak. Some monsters later in the game are capable of dropping Silver/Golden Apples, but they're usually very tough (Thunder Dragons, Mech Dragons, Silver Dragons...well, you get the idea). Silver Apples are second on Lilith's drop list, so have approximately an 1/8 chance of dropping them. So it's going to take a long time to get everyone's HP's up to the desired amounts.