There is no hard and fast rule here, in general if you feel that you're dying frequently then you have no choice but add vitality until you are able to make up for it on gear unless you subscribe to the thought that the best defense is a good offense.
Several builds suffer from this problem, demon hunters for example in top end gear feel "squishy". On the other hand even at greater rift 55 a well but not top end geared monk can survive quite well. The demon hunter might add vitality where the monk does not.
Additionally, if you've prioritized life % on gear over vitality then an investment in vitality may quickly pay rich dividends.
How much is vitality worth? At level 70, 100 life per point. source
Straight hit point numbers don't tell the whole story however. 1,000,000 hit points won't last long if you have 0 resistances thus people usually talk in effective health pool.
Effective Health Pool is calculated as TotalHealthPool / (1-Armor Reduction)(1-Resist Reduction)(1-DodgeReduction)*(1-Any other dmg reduction you want). The particular values are explained elsewhere.EHP
The bottom line is, if you don't need the vitality for a particular reason, default to main stat.