Confining a vpnd debug to one tunnel or peer doesn't seem possible, the tunnel option you are referring to looks like it is just a shortcut to execute multiple debug commands (kind of like zdebug).
If you are having problems with the vpnd.elg files rolling off before you can look at them, make sure you are only debugging IKE by just using the ikeon argument which is typically all that you need in most VPN troubleshooting scenarios. Doing a vpn debug on enables IKE debugging but lots of other debug flags in vpnd as well that will seriously clutter up your debug files. Also you won't typically need to set TDERROR_ALL_ALL which will really overwhelm your output files.
If you do require that level of debugging, you can adjust how large the vpnd.elg files are allowed to get before rolling over and/or specify the number of files to keep before they are automatically removed here:
sk112515: How to increase maximum size and number of rotated $FWDIR/log/vpnd.elg log files on Secure...
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