Oh my! This is an area I honestly can not offer any advise in.
I do not know much about Parvo or how long the disease can lay dormant on surfaces. (your yard, etc)
I am not familiar either with the length of vaccinations but I will admit, I have to question your vet not feeling your Marco is not in need of any vaccinations just because he is old.
Id honestly call a few other vets to confirm this statement from him/her. Please, in no way am I judging your vet and I hope you don't take it this way. It just sent a red flag in my mind that's all.
I hope some more experienced members here can shed some light regarding the Parvo situation for you.
Please keep us updated on what all you find out, I am very curious about this.
Thank you, and another welcome to you to Yorkie Talk.