I don’t think one can be too exacting about plan terms. Sometimes I wonder if I should be chagrined at the number of my posts that that assert something like, “the plan document sucks.”
That said, sometimes it is appropriate to be vague, though not ambiguous. The issue with an artfully vague provision is that the fiduciary responsible for interpreting the provision has to apply it and might not be cued in to the intent behind the vagueness, and eventually may have to solidify the meaning over time as the provision is consistently applied. Also, the fiduciary may be uncomfortable with interpretation, especially if the fiduciary is unskilled or inexperienced (such as “the employer”, whoever that is at the moment — a dig at those who think it is OK to designate the employer as plan administrator).
I hope you are not the fiduciary responsible for interpreting plan terms. As you describe the terms, the plan document sucks. If you are not, consult the poor fiduciary who is, as questions come up. If you are the fiduciary, consult the plan sponsor to determine the intent behind the provision, then to the extent feasible and consistent with qualification requirements and any history of interpretation/application, create a written interpretation that implements the appropriate intent and eliminates the uncertainties.