Unioning `vertices.map(_._1)` and `implicitVids` caused a strange bug where Vid2Pid would see incorrect pids when iterating through etable. Here's the version of the code with the bug: https://github.com/ankurdave/graphx/blob/MissingVertices-bug/graph/src/main/scala/org/apache/spark/graph/impl/GraphImpl.scala#L334

Oddly, if etable was forced ahead of time by uncommenting line 327 [1], the problem would disappear.

[1] https://github.com/ankurdave/graphx/blob/MissingVertices-bug/graph/src/main/scala/org/apache/spark/graph/impl/GraphImpl.scala#L327