Well, do you want to author DVDs?
If you don’t, and you have no plans to do so anytime soon, then you don’t need DVD Architect. You can edit videos in Vegas just fine without it. Used to be that you needed to have DVD Architect installed in order for Vegas to encode AC3 audio. I’m not sure if that’s still the case (haven’t had Vegas without DVD Architect in years), and even if it is, AC3 audio is used almost exclusively for DVDs anyhow.
If you do eventually intend to start authoring DVDs, you could buy DVD Architect Studio (a stripped down version of DVD Architect Pro) by itself or some altogether unrelated DVD authoring package.
Note, however, that you probably will not be able to “add” DVD Architect Pro at a later date since it is sold only bundled with Vegas Pro. I have heard of Sony cutting someone a deal if they bought a Vegas-only package from a retailer, but I’ve also heard of them declining to do. Officially, they’re probably not supposed to, but maybe some people just happen to call a sympathetic sales rep.
Rob