Hi Steve,
Unfortunately you hit a quirk of a multi-VO dirac instance. Bear with me, this is slightly lengthy:
In principle the dirac-admin-get-site-mask (no, I don't know why users should ever use a command with the word 'admin' in either) should show all sites available to a VO.
If you run this command you will notice
LCG.UKI-NORTHGRID-SHEF-HEP.uk is missing. This is because Sheffield does not support pilot roles for any VO and hence cannot run any jobs submitted via dirac[*].
LCG.UKI-SOUTHGRID-SUSX.uk is in the list. They do support pilot roles for the snoplus VO, but not gridpp, but snoplus needs all the resources it can get, so we left the site turned on [**].
We follow these problems up in GGUS tickets, but sites can drag their heels for months on end.
About the VAC sites: I can see from the admin interface that neither
VAC.UKI-LT2-UCL-HEP.uk nor
VAC.UKI-NORTHGRID-LANCS-HEP.uk run any gridpp jobs directly submitted to them. The bad thing is, I can't even tell from the admin interface if they should be able to. I will ask Andrew McN about this off the list and cc you on the email.
TLDR: In this case dirac probably does know best and won't let you submit a job at a site where it wouldn't run.
Regards,
Daniela