Equalizers do not have any effect on vibrato. As you said, it only allows you to selectively enhance or reduce specific frequencies of the sound you’re listening to. So if you set your eq to a sine wave pattern, it will just cut off and increase frequency bands alternately across the spectrum. So it will enhance and reduce the bass frequency at different points, same as with the mids and highs. In the end you’ll get a fairly flat response with some missing tones throughout.