Officially as a therianthrope Vesper must abide by some house rules for going monster mode. This includes changes to trigger a change which gets worse over time if this condition isn't cured.
To make a Player Character have werewolf stats as needed we shall be trying a modified version of the rules found in Tasha's Guide To Everything, in the Barbarian section, page 24, for “Form Of The Beast”. This seems like a decent experiment for balance. Should this character reach level 6 & this experiment has panned out well we will consider the similar “Bestial Soul” rules.
Summarized:
House Rules
To add in house rules, a werewolf PC may not use illogical skills while in werewolf mode. An athletics check is logical. An attempt to pick locks or something is not. Hit points remain unchanged. Can't speak while in form. Etc, etc.
To humor werewolf lore in this game we're going to say they are particularly vulnerable/sensitive to silver & perhaps we will find out what happens if/when silver comes into play. They do not however have pact tactics because as a player character they are not used to having a pack, so that makes zero sense for now.
In most attempts at rules for werewolves the options for a player character's belongings become to have them transform with the werewolf. This happens in a lot of movies as well. We're going to use that for simplicity for now. One moment person with gear, the next a werewolf, because simplicity in game is good sometimes. Should the werewolf revert or die, all the stuff reappears. As a fun aside however if magical items are on the werewolf at the time of transformation, the werewolf will radiate magic at an equal level as those items would should anyone be able to track that sort of thing.
In the event that Vesper should go werewolf mode and run away from The Party, this shall temporarily put the character into the hands of the DM as not an NPC so much, but rather as just the only one to secretly know the “were-abouts” of the character for the duration until the condition ends. This may interrupt the actual player by not giving them anything to do while the other players either sort it out or just decide to ditch Vesper. This is an inherent danger in playing this sort of character. Should such a thing occur, it is most likely that the DM will let the player in this case temporarily have a one shot character or control of an irrelevant NPC, a familiar or something along those lines so they're not bored.