Data processing apparatus comprises vector processing circuitry to selectively apply vector processing operations defined by vector processing instructions to generate one or more data elements of a data vector comprising a plurality of data elements at respective data element positions of the data vector, according to the state of respective predicate flags associated with the positions of the data vector; and generator circuitry to generate instruction sample data indicative of processing activities of the vector processing circuitry for selected ones of the vector processing instructions, instruction sample data indicating at least the state of the predicate flags at execution of the selected vector processing instructions.