How to mouse copy a very long line in vim terminal OS X

I want to copy with mouse one line from one vim terminal to another vim terminal. This line is huge and has a hidden character (I presume at the beginning) that I do not want to copy. So I want to copy from the first character I consider good to the last character I consider good too. The problem is the copy with mouse does not go to the end of the line because this line is huge. It goes only to the end of the terminal.The question is how can I copy with mouse from character A to character B that is not visible on the terminal ? BTW these characters are a series of visible @. One per line on many lines. But they are not lines. Using the arrow key there is only 2 lines with these @ in the middle. This shows between the first real line and the second real line.