Liriope is actually listed as a tolerant shrub to the Vantage Grass killer.
Unfortunately, once the liriope are up and thriving the only way to kill it is to carefully spot treat with a glyphosate product such as Agrisel Gly Pho-Sel Pro. Agrisel Gly Pho-Sel Pro is a non-selective herbicide which means it will kill most types of vegetation including desirable turfgrasses, so care must be used when applying this product to the turf areas.