Should I pay half of my first salary or half of every salary after vowing to do so, and what if I don't get a job and work on a daily wage?

The foundation of vows lies in the intention of the one making the vow. If the intention is absent, then the rule is based on custom, followed by the implication of the wording. If you intended to donate half of your first salary, or if the custom regarding your vow specifically refers to the first salary, then you are only obligated to donate half of it. As for daily wages, they are not included in the vow unless you intended them to be, because both custom and wording specifically refer to the salary associated with a regular job, not with daily work.