I own VWITX (same fund as VWIUX but investor shares) and Wellesley.
It's hard to compare Wellesley and VWIUX because they are 2 different beasts: one is 100% municipal bonds with high credit ratings, the other a mixture of stocks and taxable corporate bonds. So there is no doubt that Wellesley is more "risky" than VWIUX, though I would suggest that you include inflation risk in your analysis. In a taxable account though, VWITX would be more tax-efficient than Wellesley. VWITX is tax-exempt at the federal level, Wellesley is notoriously tax inefficient (large taxable dividends, mostly unqualified, and sometimes large capital gain distributions). But tax-efficiency is not the only thing to consider: you have to try and maximize your after-tax returns as well.
If you are in the 15% tax bracket, it might make sense to own taxable bonds in your taxable account. That's because, in a normal environment, tax exempt bonds pay less dividends than taxable bonds. As a general rule of thumb, the break even point is generally around the 28% tax bracket. In other words, if your marginal tax bracket is lower than 28%, you would get a higher after-tax return with taxable bonds. If your tax bracket is 28% or higher, you would get a higher after-tax return with munis. Of course you have to look at you own tax situation and decide for yourself which type of bonds makes more sense for you.
42 y/o, married, retirement portfolio = 43 x annual expenses