The vaitālīyam and aupacchandasikam are two closely-related meters that combine the principles of the vr̥ttam and jātiḥ classes: they are mora-counting at the beginning of a verse line, and syllable-counting toward the end. They are probably “transitional” forms, since they do not occur in the oldest texts (which are dominated by vr̥ttam forms) and are very rare in classical texts (where jātiḥ forms occur freely); you are most likely to find them in the canonical and postcanonical literature of the Buddhists and Jains.
They are ardhasama meters, which means that they are divided into four quarters, with the two even quarters (the first and third) and the two odd quarters (the second and fourth) different from each other in length. The vaitālīyam has 14 moras in its odd lines and 16 in its even lines. But the last eight moras of every line is taken up by the same syllabic pattern: ऽ।ऽ।ऽ. The aupacchandasaka is identical, except that it has an additional heavy syllable at the end of each syllable. Therefore it has 16 and 18 syllables in its odd and even lines, respectively, and both end with the fixed cadence ऽ।ऽ।ऽऽ. Any configuration of syllables can come before the fixed cadence.
Piṅgala, Chandaḥśāstram 4.33–34
vaitālīyaṁ dviḥsvarā ayuk pādē yug vavasō ’ntē rlgaḥ g aupacchandasikam