“Vein” can mean “quality, style, tendency, manner”.
I agree with Dan Smith that it’s not perfectly clear what it means. Perhaps it would be clear to someone who is familiar with this subject matter, but I’m not.
I know who Guevara and Castro are, but I don’t know much about exactly what they believed and how they viewed themselves.
If Dawei Zhijiage’s interpretation is correct and it means “Some Marxist terrorists saw themselves exactly like Guevara and Castro”, I’m still not sure what that means. Like them in which way? That they were the manifestation of the proletariat revolution in their countries? Perhaps, I don’t know.
An easy to understand use of “fully in the vein of” occurs in this example:
“By the mid-1950s, she was working fully in the vein of Abstract Expressionism.”
Here you can simply replace "vein" with "style".