Auto-increment columns do not work very well for sharded tables. Vitess sequences solve this problem. Sequence tables must be specified in the VSchema and then tied to table columns. At the time of insert, if no value is specified for such a column, VTGate will generate a number for it using the sequence table.
Vitess also supports sequence generators that can be used to generate new ids that work like MySQL auto increment columns. The VSchema allows you to associate table columns to sequence tables.