You can can create a subset using a polygon in WKT format.
You could also mask the data within your vector using the Land/Sea mask operator. It can take as input a vector found in the source product. This would make anything outside your vector no-data-value.
If you source product does not have the vector you need, you can use the vector/import-vector operator to add a shape file to the product.
Subsetting directly from a shape file or source product vector would be useful. I’ll add it to the list of future enhancements.