The Vine Heart is an "AnySkin" item, meaning that it will always use your avie's current skin for it's coloring. In particular, it uses the Tongue. Sometimes the tongue of certain monster skins can be radically different colors, but in most cases, it will be pink. Tongue coloring is often an afterthought for developers, so if you don't like the color, try out some other skins and see what their tongue color is.