Your medical/dental coverage is paid at "employee active rate" as the VPLOA still makes you ACTIVE on the seniority list with all rights to sick accrual/vacation etc...intact.
Your 15th pay check, which is the "clean up" from your flying activity from the previous month will have the MED deduction at your regular active rate. NO ONE should be billed for a full premium and your medical coverage should have NO disruptions. If you take this type of leave every month that it is offered, the company may bill you at the employee "active rate" since you have no earned income from the company while on this leave.
The VPLOA (Voluntary Personal Leave of Absence) is contractual and has been in the contract for at least the past 5 agreements section 20-1 A.2.
The VPLOA is NOT a furlough, but rather a "Leave of Absence" offered on a month to month basis depending on the staffing needs of a particular base, and your status remains "ACTIVE" for every month it is offered and you are awarded it. It is also awarded by seniority on a voluntary basis.