All vitality until 50 and you start acquiring gear. Then a decision based on your stats can be made. If you are sitting in range for the next strength tier to increase block strength, that is probably worth more than the extra vitality. This of course will depend on what you are doing, as some fights the extra mitigation won't help against heavy burst damage. A bad example that shows the point would be Titan HM mountain buster, it it one shots you extra block strength won't help as much as some extra vitality that might make you able to survive.