Same. Seeing how supportive Julia is for her daughter, i can totally imagine Julia telling her daughter to dress like that.. but that usually only works if Vik would like to impress the mc + she would need to know that he will be coming.. otherwise it wouldnt make much sense yet..
Julia could be trying to sabotage and wash all of viks clothes, leaving only c outfit, buuuut that doesnt sound believeable either, haha...
She could resort to emotional blackmailing: "i was looking around for hours/days for your outfit, please PLEASE wear it for your birthday for me.. pretty please?"
...
another idea could be that Julia tries to trick Vik by taunting her: "im not sure this outfit would look any good on you.. i think you became a bit.. fat?" - making Vik accept that challenge and tries it on, but the moment she wants to change back, mc arrives and tells her she looks really good on that, making her change her mind in a tsundere way... who knows xd