I was talking to a woman recently on the matter of penis size. She was confirming that yes indeed she and pretty much every other woman she knows much prefers a modest size penis and that large ones are just uncomfortable or painful when having penetrative sex. What matters far more to the pleasure of sex is the confidence of the owner of the penis than its actual size.
To that end the discussion turned to vibrators and an experience that she had with a man who while he was reasonably confident with his penis he was vastly threatened by her owning a vibrator! He felt that it was an attack on his prowess as a lover.
So lets be really clear guys – you need to let that insecurity go. A woman you are dating would prefer to get her pleasure from you rather than a device if at all possible. Just like you would rather receive pleasure from that woman than masturbate alone.
Everyone responds to sexual stimulation differently and has their own preferences. If a woman needs or just enjoys stronger clitoral stimulation than you can provide, then that is just a fact of who she is and not a failing on your part. If she wants to use a vibrator while having sex, then treat that as just a part of the fun and find ways to make that work for both of you.