Why store a vendor ID in PO items anyway?
(OK, that is intended as a rhetorical question, but if you really think there is a good reason, please tell us.)
In this common setup
Vendor --< PO --< PO Line Items
the vendorID is stored with the PO itself, and since each PO item belongs to (exactly) one PO, it “knows” that PO's vendor, and by extension, the name – which also means that you can simply display that ID (or name) with every PO item (or, more commonly, once for an invoice or other line-item based correspondences).
without using script.
You can't – unless you define an unstored calc field that simply references the vendorID field from PO, but that makes sense only in very few scenarios. If you'd want to store the ID in line items you need a script – auto-enter calcs and lookups cannot be triggered from field changes in related tables, only from “native” ones – or with a script.
I need this logic because due to custom requirement of project or reporting purpose or value list display from line items instead of PO(Can't make a value list without storing Vendor ID from line items, anyway).
I got your point but i am finding another way to do this thing.
Isn't the vendor ID still a usable related field To the line item?