QB Issue Resolution:

Other than if you use time, there is no way to enter the data before you, well, enter the data (on a paycheck.)

You can either keep track of this on a worksheet, or you can start payroll early and enter the split between regular and vacation, and then save for later.

Resolution for Issue 'How can I enter future vacation requests for an employee so that when I process payroll it will already be entered as vacation hours for that week’s paycheck?' available: Yes (Solved).
Source: Intuit Community forum.

How can I enter future vacation requests for an employee so that when I process payroll it will already be entered as vacation hours for that week’s paycheck?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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