An amount of € 1,500 is secured with the VPI (2015).
The starting month / year is December 2019.
What is the amount in December 2020 (= comparison month)?

You need the values ​​of the index series 2015 for the
both months of December 2019 and December 2020.

The current tables can be found on the Internet …
for Germany: Homepage of the Federal Statistical Office
for Austria: Homepage of Statistics Austria
for Switzerland: Homepage of Switzerland Statistics

Required data (example AT):
Value for December 2019: 108.1
Value for December 2020: 109.4

Calculation:
Step 1: 109.4: 108.1 = 1.01203 (factor)
Step 2: € 1,500 x 1.01203 = € 1,518

Old amount: € 1,500
New amount:** € 1,518**