1. An apparatus for executing an all-to-allv operation on a parallel computer that includes a plurality of compute nodes, the apparatus comprising a computer processor, a computer memory operatively coupled to the computer processor, the computer memory having disposed within it computer program instructions that, when executed by the computer processor, cause the apparatus to carry out the steps of: packing, by each task in an operational group of tasks, vectored contribution data from vectored storage in an all-to-allv contribution data buffer with a variable data element size into an all-to-all contribution data buffer with a fixed data element size, wherein two or more entries in the all-to-allv contribution data buffer are different in size and each entry in the all-to-all contribution data buffer is identical in size, wherein packing includes: scanning by each task the all-to-allv contribution data buffer for maximum all-to-allv data element size; communicating, by each task, each task's maximum all-to-allv data element size to each other task in the operational group; and setting a cell size in the all-to-all contribution data buffer to the maximum all-to-allv data element size; executing, with the contribution data as stored in the all-to-all contribution data buffer, an all-to-all collective operation by the operational group of tasks; and unpacking, by each task in the operational group of tasks, received contribution data from the all-to-all contribution data buffer into the vectored storage in an all-to-allv contribution data buffer.