Congrats on your successful first outing!
The vent is supposed to be covered... if you can see daylight, then you'll also get wet when it rains!
Your '67 is in between my experience with a '61 and a '70, so I'm not sure when they switched to the newer style. The older trailers had a metal cover with two actuators that allowed for either or both ends to be open. Those actuators are no longer available, however I saw on another thread here that someone is making a very nice reproduction. My '70 also had a metal cover, but used the later, and now more common design: a single crank actuator that only opens the cover to the rear.
I'm sure that someone with a '67 can tell you which yours was. I CAN tell you that a replacement cover will be expensive. Andy at Inland had some plastic vent covers made a number of years ago that fit my '61. He had them made as an insulated light dome; a very nice design and fit, but as I recall, it was around $100 shipped.
I think that if I had to replace one now, I'd just install a Fantastic vent fan.
AIR 2053 Current: 2006 Born Free 32 RQ Kodiak Chassis, & 1995 Coachmen B-van
Former Airstreams: 1953 Flying Cloud, 1957 Overlander, 1961 Bambi, 1970 Safari Special, 1978 Argosy Minuet, 1985 325 Moho, 1994 Limited 34' Two-door, 1994 B190 "B-Van"