I think it's fine putting all your money into the target date Vanguard fund within your 403b because it has one of the lowest expense ratios available in the company-sponsored plan ... but, to me, the question is whether you should transfer the existing balances in your IRA / Roth IRA into different funds for purposes of diversification.
IMHO, the target date style fund has a commonly used/accepted asset allocation and is designed for easy/lazy investing. If you're happy with set-it-and-forget-it, then that may work for you. I personally prefer to spread things out a little ... adding more international and small caps, for example ... so that large-cap domestic isn't so overweighted. I do this within my Vanguard Roth IRA since I have plenty of choices there (as opposed to the limited selection offered by an employer).
I don't believe that either strategy is inherently "wrong". It is a matter of opinion and personal preference. Whatever works for you.