Here's the best info I could figure out:

There is some discrepancy with Vicky's age because in ep 7 she says that a picture of her as a baby was taken ("in the same time frame)" ten years before when electricity was introduced to the village by Brian Roscoe (1914). That may be a translation issue it might not mean "taken" in the sense of making a photograph, but placed in the box.
Given the following you could argue that her age is anywhere in a range from 10-15.

Based mostly on the "Grey Wolves" story and the "Queen Berry" we've got the following dates:

1904 Cordelia accused of murder (20 years before the present, Ep 8).
Cordelia is 15 and is thrown out of the village

19xx Cordelia raped, gives birth to Vicky and is sent to an insane asylum(Ep 19)

1914 Electricity introduced to the village by Brian Roscoe (10 years before the present, Ep 7)
The original "Running of the Hares" is spring(?) on the Queen Berry (Ep 3)

1914+ Cordelia meets the girl, Julie, from the Queen Berry who has the heart shaped locket. (Ep 19)

1914+ Cordelia escapes with Brian's help (Ep 6)

1914+ Photo left in box and headstone marked by Cordelia(Ep 7)

The Queen Berry incident had to be early in the spring of 1914 so Vicky must have been born no later that 1913 if Julie was just admitted to the asylum when Cordelia met her. That would make the minimum 10 yrs.

1914 approx Cordelia disapears 16 years prior to the discussion Kujo has with one of the Coco Rose actresses (Ep 20) and Victorique was born one year after she was kidnapped by de Blois. (Ep 19). The discussion was in the winter of 1924-25 after Victorique's birthday. So sixteen years would place the date sometime in the 1909 timeframe. (h/t to EyeofPain for pointing this out.)

Brian appears older when he helps Cordelia escape than when we see him in the dancehall watching and has money since he was able to fund the electricity at the village. He says "sorry I took so long". Which could either be ironic or may have been a number of years, but based on the 1909 date, Cordelia was held for 4-5 years before escaping. That would jibe with Victorique's apparent age when she visits her.

Some of the wiki's give her birthday as Dec 25, 1910. This would appear to make sense with the other information +/- a year.