Log into your vanguard online account. There will be an option to adjust the percentages withheld from your monthly check into any of your accounts. Just add the after-tax percentage you want withheld.
Your preTax contributions you don't pay income tax on. Your post tax contributions you do. Kind of like a non deductible IRA. But they grow tax free (like an IRA or 401k) until you withdraw them. Hopefully in retirement and at a lower tax rate. It may not be the best use of available dollars for everyone. But it is an option.