Sample preparation methods are typically very specific for the type of sample and HPLC method/detection method used , Please have a look at the following review, it is a comprehensive review of sample prep and analysis methods of vitamines with (mainly) LC in a wide variety of samples (including plants).
Article A Review of the Extraction and Determination Methods of Thir...
In this application note of Thermo you will find a table with Solubility and stability info of a variety water- and fat-soluble vitamins. Which could be helpful.
Combined analysis of Fat Soluble and Water Soluble vitamin analysis (at low concentrations like in plant samples) by HPLC is rather difficult. Please check this review. https://www.chempap.org/file_access.php?file=593a202.pdf
Or do mean a combined work up, with separate analysis for FS and WS ?