Pipelined requests are possible in Vertica as long as you have a projection that is already sorted on the column you have as your predicate , the optimizer will use the faster GROUPBY PIPELINED operation. This shows up in your explain plan as GROUPBY PIPELINED, see example here:
=> EXPLAIN SELECT COUNT(DISTINCT annual_income) FROM customer_dimension
WHERE customer_gender = 'Female';
Access Path: +-GROUPBY NOTHING [Cost: 161, Rows: 1] (PATH ID: 1)
| Aggregates: count(DISTINCT customer_dimension.annual_income)
| +---> GROUPBY PIPELINED [Cost: 158, Rows: 10K] (PATH ID: 2)
| | Group By: customer_dimension.annual_income
| | +---> STORAGE ACCESS for customer_dimension [Cost: 144, Rows: 47K] (PATH ID: 3)
| | | Projection: public.customer_dimension_DBD_1_rep_vmartdb_design_vmartdb_design_node0001
| | | Materialize: customer_dimension.annual_income
| | | Filter: (customer_dimension.customer_gender = 'Female')