Probably the best would be grinding the plants with some water-methanol or water ethanol and than using either liquid-liquid extraction with e,g, hexane. Alternative would be to used solid-phase extraction with C-18 of the aqeous mixture after filtrtation of the course matter and extraction the vitamin with e.g. hexane from the cartidge,

After that use HPLC with UV detection as decribed in pharmaceutical literature, Pharmaceutical literature e.g. vitamin analysis in tablets may also be an inspiration.

Agree To Jos

Attaching article here which describe HPLC method for water soluble vitamins. You need to modify sample preparation method and it is very similar  to the method adopted by  food industry.

Good Luck.

Thanks.

Thanks all for your answer