What procedure would improve the definition in my chin and the look of my neck? (Photos)

It does appear that you have relative microgenia (short chin). I would recommend an anatomic chin implant that will strengthen your jaw line and improve the appearance of your neck/jaw angle. Typically, liposuction of the neck is performed at the same time to achieve tightening of the underlying skin which will contour better to your jaw and give you more definition. We often perform these procedures in conjunction to give the best results. I would seek consultation with a board certified facial plastic surgeon in your area.

Hello Cybervegan,
I just scanned through all the answers on here provided by other surgeons -- and if I were you, I'd be super-confused!
Let me clear this up.  80-90% of your concern will be alleviated with an extended anatomical chin implant.  This type of implant will bring the chin forward, giving you a more defined neck angle, and fill in the pre-jowl area to decrease the appearance of your 'marionette' folds.
Do not go for a surgical necklift.  Some amount of neck liposculpting may be necessary to loosen the skin for placement of the chin implant, but I would even recommend against any significant liposuction of the neck fat.
That being said, this is not something you NEED to do.  Just KNOW that you are beautiful the way you are, inside and out.
Best wishes on your journey,
Dr. Jay