Is it as effective if I do one vial of Kybella per treatment instead of two per treatment?

This will depend on the individual and needs an in-person evaluation. Both Kybella or neck liposuction are good options and will improve the double chin. It depends on your lifestyle and personality which would be better for you. 
Kybella is best for patients that:
1)  Do not want surgery 
2)  Are not surgical candidates 
3)  Not willing to wear a compression dressing around the face for 1 week 
4)  Patient-  The results of Kybella will take months
Kybella take 1-3 treatments every 6 weeks.  Patient will develop swelling and possible bruising after each injection that may last a few days.  The benefit of Kybella is each treatment is quick and completed in the office. However, some patients need more than one treatment to get desired results. 
Neck Liposuction is best for patients that:
1)  Want a single treatment 
2)  One recovery time-  Downtime for neck liposuction require wearing a wrap around face for 1 week. 3)  Less painful 
4)  Quicker results 
5)  Cost is a concern
Neck liposuction is a quick surgical procedure that can be complete while the patient is awake.  It cost around the same price a Kybella and can be less in patients with heavier necks.  The single recovery of neck liposuction may be a better option for some patients.
I hope this helps. 

It is effective to do one vial only but it may take a very long time to give you the results you want.  Normally, it takes about 2 vials per treatment and everyone needs a minimum of 2 treatments to achieve results.  The benefit of one vial is that you will experience less swelling per treatment

It is never a problem to do one vial instead of you.  You might need more treatment sessions if you do it one vial at a time.