I am 99.9999% confident there is no purpose for a vocative gerund. Yet nothing seems to specifically disallow for such a construction. In theory something such as "odi te currendum" (in English, "I hate you, running!") should be perfectly acceptable, albeit strange, ridiculous, and unnecessary. But is there ever a recorded instance of such a construction?
I've never seen the gerund used in the vocative, and a search for -ende in the Packhum corpus turned up nothing but imperatives. But I would be very surprised if such a form existed.
The gerund in general is defective, in that it has no nominative. If this missing form is needed, it's replaced by the infinitive.
Since the vocative is almost always identical to the nominative in Latin (the only exceptions being the masculine second declension, and a few scattered foreign words like Iēsus), I would expect the vocative of the gerund to also be identical to the nominative: that is, nonexistent, and replaced by the infinitive.