Could you give a brief description as to what it looked like when compared to the Griffons? Think Ruppell's are about 10% smaller & have darker coverts.
Caution is needed in identifying Ruppell's, because some griffons exhibit coverts as dark as many Ruppell's.
The size difference may be noticeable between two adjacent vultures, but with birds separated by any distance or in a single bird it isn't obvious.
This vulture, photographed in 2012 near the Strait of Gibraltar, with its dark and un-contrasting plumage and clean edges to the very pale under-wing marking was originally identified by myself and later by others who are very familiar with the species as a Ruppell's but further examination showed it to be a dark griffon. I've seen quite a few real Ruppell's on my regular autumn trip to the Strait, but I now look at them extra hard.
Further inspection shows that although its dark plumage and darker head looked good, the head is actually too large, the wing (arm/hand) proportions don't look right, the body is too bulky and the tail looks too long.
This photo was actually used as an example of a confusion species for Ruppell's in an article (in English) in Dutch Birding (Vol38, No 6) by Javi Elorriaga and Guillermo Rodriguez 'Identification of Ruppell's Vulture and White-backed Vulture and vagrancy in the WP'.
EDIT. A bit of additional information. At first I took the additional white bars on the underwing to be a sign that it was a 2nd year bird, because information was limited beyond saying that additional white bars develop as the birds age beyond the first year birds I'd seen until then. In fact close inspection shows the white ruff of an adult griffon.