Is it possible to have different voices playing with different dynamics on the same staff?

In my example (attached), I'd like to have one voice for the vocal, and a second voice played a bit softer for higher instrumental notes.

You'll see on the attachment, the first 8 measures look clean. But when the vocal part begins on the 9th I've moved them all to the bass staff. It plays back the way I'd like, but aesthetically the bass staff just becomes too jumbled to read easily. And I'd like to avoid adding a third staff for the vocal, if possible.

Thanks!