If you're just talking about a regular taxable investment account than it doesn't matter at all and one account for both of you is fine. Maybe in the case of a divorce or something it might make a difference, I don't really know, but I would expect it wouldn't.
If however you're talking about having set up an IRA (Individual Retirement Account) with Vanguard and you are currently contributing the maximum to the one you have then it makes sense for you to set up a separate one for your wife. The yearly maximum for contributing to an IRA is currently $5,500, but it's an *Individual* retirement account so you can both have one, which doubles the amount your household can contribute to $11,000.