I could find no conjugation of any close verbs that include vorlagen. I found somewhere a mention of a subordinate clause form of vorliegen, which I had never heard of before and can find no reference to in any of my books.
The clause in question is:
Da bis zum Zeitpunkte der ersten Beobachtungen von Feindhandlungen weder besondere Feindmeldungen vorlagen, …
The rest of the sentence is quite long but if needed for context I can add it.
Clarification on the meaning/form of vorlagen would be greatly appreciated as well as direction to information on subordinate clause forms of verbs (that apparently are not included in the conjugation in 501 German Verbs) would be greatly appreciated.