Hello, how can we set subsidiary list as ascending order to the subsidiary filed in the vendor record? does it need to any customization for this?

Not 100% sure what you mean, but it sounds like something that is trivial that is just visual & doesn't really matter? Can you post an image as an example?

We have lot of subsidiaries in my account, I am able to see list of values in the subsidiary field but there is no order wise I need to show subsidiary list in alphabetical order.

Probably not possible. If there is an order I bet it's by internalid. It's just a nice to have really?

Is it possible to add those values to another multi select field in order wise? Is it possible to any other customization?

Order is by internal ID of parent. We have an account with 165+ subs and it is murder

Does creating a vendor or vendor-subsidiary saved search help? Sort it by vendor then subsidiary field then add an Available Filter