While I agree that the attribute-system may be borrowed from Ultima 7, one has to keep in mind that the two games were developed simultaneously. It may have been a mutual design decision, or a case of one team 'stealing the test answers' from the other to get an attribute-system going. We may never know.
Regarding Vitality, you may be on to something there.
I looked in the official clue book, and they list the three attributes and how they relate to each of the classes, as well as their value. Ranger has Strength, Dexterity and Intelligence all at Medium, meaning they start at an average number, but then get an average modifier applied to them to get the final stat value. Shepherd meanwhile has them all on Low, but then gets massively bigger modifiers applied. No class has all three at High, but the other six classes have a mix of High, Medium and Low. This information is published in the clue book to help players get their preferred character 'build' going. For those who want a beefcake character who can still use some magic, for example, the Druid is the best choice.
As you can see, Vitality isn't mentioned once. It appears (I haven't checked) that both Hit Points and Mana are derived somehow from the three basic attributes, but then the Mana-skill can be used to raise Mana further, while Hit Points are increased by gaining levels.
I recognize that sometimes, during game development involving complex systems, that things don't always follow the same rules. In the more modern survival horror game The Long Dark (to name but an example) there are nine core skills which increase via their usage. Use the bow to hunt wildlife, and the 'Archery'-skill increases. Successfully boil water without hurting yourself, and the 'Cooking'-skill increases, etc. These skills follow a level system (1-5) where each level gained earns neat bonuses, and they can also be trained further by reading skill books that can be found. However, the game also has three secondary skills which don't follow those rules. While these secondary skills do increase with usage, they have no level system but have an 0-100 attribute system instead, with 100 being the maxed-out skill with the best benefits... but no bonuses are involved at any point, and no skill books are available.
I believe that something similar may be happening under the hood with the Underworld-games.
