I will make two contributions for the entire year to my Vanguard solo 401k for both the employee and employer. How do I record the 401k bank deductions within Quickbooks?

Hey there, @2005006.

I'll help you set up a payroll item for 401K to record both employee and company contributions in QuickBooks Online (QBO) Payroll. Once you run a payroll, this will automatically record the deductions into your bank account. Here's how:
 

1. Go to Payroll, then select Employees.
2. Click the name of your employee and select the Pay pencil icon.
3. Go to Does employee have any deductions? section and click + Add/edit deductions.
4. Select +  Add/edit deductions/contribution.
5. From the Deduction/contribution ▼ drop-down list, choose + Add deduction/contribution.
6. Select a Deduction/contribution type and Type. 
7. Add a Description. This is the deduction or plan name which appears on paychecks.
8. Choose whether you want the deduction to be calculated as a Flat amount or Percentage of gross pay. Then enter the amount or percentage. 
9. Click Save, then Done.

For more information, as well as how to assign retirement plan deductions and company contributions, I recommend the following article: Retirement plan deductions/contributions.

Drop me a comment below if you have further questions about setting up 401k or other related queries when adding payroll items. I'll be happy to help you some more.