To send out a reminder for only volunteers who are scheduled in the next week, two weeks, etc, start in the Announcements pane.

The best way to control the recipients in this scenario is to use the {{ScheduledShifts}} token.  This token will display shifts for a specific time frame of your choice, as well as exclude volunteers from receiving the email who do not meet the criteria (who are not scheduled or the specified time frame).

In the "To" field make sure it says "All volunteers".  Next, setup the content of your email.  You can use the default template provided when VSP is first launched, or type up your own note to the volunteers.

Click on the Token menu, and go to the section for "Assigned Positions" - then choose the {{ScheduledShifts}} token.

After hitting "Preview", the Token Options window will open.  This screen is where you will specify the time frame of shifts you want to display.  For example, "Include all scheduled shifts from today forward", "Include scheduled shifts in the following "x" weeks only" (where you can change the number of weeks), or "Include scheduled shifts in the following period of time" where you can specify the block of time.

You can further choose to specify to only include positions for a certain job or shift.  Make sure in this section to check the box that says "Completely exclude volunteers from this email or roster who are not scheduled for any position that matches the given criteria".  This is what will act as the filter when the emails go out.

Once you are satisfied with your email, click the "OK" button and VSP will indicate how many volunteers you will be sending the email to.