I think how I'd set this up depends on what your environement looks like, and your hard and fast requirements.
We haven't experimented with synthetic very much, we've been just fine with weekly full and daily incrementals. Fulls to us are the most important so we don't even bother sending incrementals off.
First of all, do you have a lot of change? If so, then Veritas points out that synthetics won't really help you that much.
Are you hard and fast on putting the data to tape daily? If for instance you could send the data to the DR site weekly, you could run your full, then your week's worth of incrementals to your disk, then the day before you're supposed to ship it to the DR site, go ahead and run the synthetic schedule, duplicate that synthetic full, send it off, run your incrementals to disk through the week, rince, repeat. You have your copy on disk for quick restore, have your copy offsite, everyone is happy. If I could only send offsite once a week, that is what I'd do in your case if the change is low.
You could still do the synthetic thing with a daily vault. Just just send the synthetic off in place of the weekly run full. If the change wasn't too high, that is probably your best bet if you have the processing time and power on the server to make the synthetics every week from the disk, just to help save the disk and time on the client, you won't save a lot of money on tapes though, as you're still sending the full tape off weekly.
If your change is high, I'd probably go with whichever is less for you, daily fulls, or weekly full with daily differential incrementals.