Mona vs. Billy and Wyatt
The similarities of Vagabond and Easy Rider pretty much stop at the fact that both groups/individuals are outcasts that are in search for something. However, Wyatt an Billy seem to have and advantage over Mona in the fact that they are "independent" in their travels. They have their motorcylces to travel, whereas Mona has to depend on other people to travel. This could infer that women are inferior to men because she has to rely on others, which fits a common stereotype of women always needing protection.
Billy and Wyatt also seem to have and "easier" time getting places because they are male. Even though they aren't accepted in normal society, they are less vulnerable because they are 1. in a group and 2. they are male. Mona being by herself and a female is more "in danger" as in the scene with the purple guys dressed as trees.