It doesn't matter if it is NS or Sterile Water. This is extremely old thread, and those were probably the initial instructions a long time ago.
The differences that you will see with repeated dosing in the same patient is usually d/t context sensitive half lives.
"Infusion solutions of vecuronium can be prepared by mixing vecuronium with an appropriate infusion solution such as Dextrose Injection 5%, Sodium Chloride Injection 0.9%, Dextrose (5%) and Sodium Chloride Injection, or Lactated Ringerís Injection." http://www.globalrph.com/vecuronium_dilution.htm