Would Vivace take care of my mild double chin appearance? (Photo)

Vivace doesnt help double chins. most younger patients who want substantial results do facetite and/or threads and then maintain with kybella, morpheus 8, and thermage/ultherapy.
As we age the skin sags there is  nothing  you can do that but try preventative  energy-based or thread-based  treatments or get a neck/facelift earlier in the aging process.  There are numerous options and some can be used in combination to get best outcomes but you need a consultation with an expert.
Options include for skin tightening :

thermage radiofrequency to heat skin and tighten it over time and prevent the aging process
ultherapy highintensity ultrasound to contract muscle and tighten the face and neck and prevent the aging process
PDO nova threads or instalift threads for immediate lifting without surgery and long term collagen production
sculptra injections for collagen production to areas of the face and neck with crepe skin or early sagging
btl exilis ultra, venus legacy, thermi250 , or inmode forma/contoura  for constant collagen stimulation monthly with facials
facelift/necklift with fat grafting/fillers if you need a big improvement
ThermiRF or J plasma /Renuvion or Inmode FaceTite   internal heating and tightening to long term internal heating and tighting with or without surgery
Platysmaplasty to tighten the muscles of the neck long term and create contour
Facial implants or fillers to contour the jawline and chin to improve the apperance of the nec k
Fractora/Morphenus8 or Intensif microneedling RF for improve superficial tightening, neck and chest wrinkling and crepe skin 

You need to see an expert in facial sculpting and age prevention.  Best, Dr. Emer.

Great question.  You may benefit from some tightening of the skin with Vivace, but not likely  on the neck  where you are concerned. A true profile view photo (whole body turned, not just head) would help, but an in person consultation (once COVID crisis is over) would be even better.  You seem too young to benefit much from anything, though Coolsculpting or Kybella might help that little bit of extra fat under your chin.   Be well.