Yes vinegar will indeed kill off weeds as well as grass so if you want to kill weeds and not the grass then pulling the weeds in the grass area is a better option or use some weed killer that does not kill grass.
Vinegar is a natural non toxic weed killer that works great for killing weeds but you don't want to use the vinegar to kill the weeds when it's on grass you don't want to die.
If you want the grass to thrive but the weeds to die instead then either just pull the weeds out by hand or use some weed killer that doesn't kill off grass.