Collagen vacscular disease is not a specific disease but a category of diseases like dermatomyositis, scleraderma, and rheumatoid arthritis. There are others. Here is a link that explains it.
UCTD is an overlapping connective tissue disese that is not entirely defined. It will usally develop into a clearly diagnosable disease in time. MCTD (mixed connective tissue disease) is my affliction and it is a combo of lupus, scleroderma and polymyositis. Scleroderma is a collagen vascular disease that deposits collagen in tissue. It most commonly affects skin but it can affect any organ in the body. It is a process that is tantamount to scarring and there is no direct treatment once that happens. I probably have some in my digestive tract and esophagus. It was feared that I had problems in my lungs but tests showed that they were OK.
Lupus is another AI disease and is a different disease. In my case sle is part of mctd but I also have symptoms of scleroderma and polymyositis.
All of this is confusing to understand and each of us manifests our particular disease differently. Lupus has been called the disease with a 1000 faces due to it's diverse nature and symptoms that often mask those of other diseases. It can take years for some lupers to get a positive diagnosis. MCTD is much rarer and more complicated....I refer to it as the disease with 10000 faces. Many people with AI diseases will have symptoms, even if not diagnosed, of more than one disease.
This makes sense in a way. Our genetic makeup is susceptible to a trigger. Depending on the trigger (most of us will never know what that is for sure) we can develop one or another disease and often develop more than one. Our immune systems are out of balance and misdirected. AI diseases run in families but not the same ones. One will have lupus and one will have RA, and another will have DM or PM. But the underlying problem is the same...a susceptible immune system due to our genetic inheritance.
Hope that makes some sense of the mystery. Ask more questions if you are still confused. There are lots of good resources on the internet as well.