Well, if the item isn't properly cleaned you can get infections.
Some women report they lose sensitivity and get to the point where they've inadvertently trained their bodies to only be able to orgasm with a vibe. I'd recommend switching it up. Use different settings in different positions, use your hands sometimes, have oral and PIV with your partner, etc.
Just like anything, moderation.
How vibes impact relationships depends. For women who cannot orgasm without a vibe, using one is necessary for her sexual satisfaction. Some men don't mind, some do. Of course, some woman will refuse partnered sex because they are meeting their own needs, so making sure to take into account your partners needs is important.