I like roof vents over ridge vent. On my rental property when I roofed it in 07 I replaced the ridge vent with Lamanco's.
A good friend of mine did the same thing as well. He's been building high end homes for over 20 years. He said that he didn't like how much heat escaped his lake home so one day he cut holes for Lomanco's and couldn't believe how much hot air poored out of the vent openings. He assured me the felt was cut out of the peak.
My father seen with his own eyes 3ft snow drifts in an attic from ridge vents.
As far as 50ft of ridge I'd go with 9 vents. Odd is always easier than even.
As far as vaults you have to do ridge vent, there is no other choice.