Is the VTI already in the Roth IRA? If not, keep tax considerations in mind - IRA contributions can only be made in cash, so you'd be selling the VTI, then contributing the $3K to your Roth IRA, then buying the VTSMX so you can convert the whole amount to VTSAX.
If you're OK with any tax issues related to the sale, and you still have room to contribute for your 2018 Roth IRA, then yes, I'd do that.
If the VTI is already in the Roth IRA, then I'd just go ahead and do that - simplicity and all.