Do greater blocks of european vacation time create opportunities for european entrepreneurs to conduct R&D for themselves (for new ventures) while working with their existing employers?

It potential could but I have not found that this is the case at all.  It has to do with the cultural priorities where down-time with family is taken very seriously and is sacred therefore when on holiday people completely turn-off.   This is a generality so it is possible to find examples where this is not the case, a young person who is not in a committed relationship and is living away from their immediate family.  For example, if someone decides to move to Berlin from another city and they work for a company but they have a focus to be there to create a business themselves.  These are the exception, not the rule.

The broader question is one of whether a social safety net, regulated labor conditions, can foster entrepreneurial activity when the circumstances leave time to work on goals that are beyond the primary employment.  I have observed that this seems to be the case in some countries but it is more a function of adversity versus opportunity.  Adversity is a strong motivator, often much more than opportunity.