I have two tables:

PART
-----------------------------------------------------
PART_KEY - INT (PK)
PART_NUMBER - VARCHAR(100)
VENDOR_NUM - VARCHAR(100)

and

BOM_PART
--------------------------------------------------------
BOM_KEY - INT (PK)
VENDOR_NUM - VARCHAR(100)
VALUE - VARCHAR(100)

PK = Primary Key

I am using Hibernate2 and want to create a mapping of many-to-one in BomPart.hbm.xml such that I will have the Part matching the vendor_num of the BomPart. How can I do that.

vendor_num in Part table is not defined as primary key but is unique?

The example in section 6.1.14. properties in the reference documentation may help you.

I am using Hibernate2. Can this be accomplished with Hibernate 2?

Here is the section from the H2 reference document that I think is relevant to your problem (from 5.1.10. many-to-one):

The property-ref attribute should only be used for mapping legacy data where a foreign key refers to a unique key of the associated table other than the primary key. This is an ugly relational model. For example, suppose the Product class had a unique serial number, that is not the primary key. (The unique attribute controls Hibernate's DDL generation with the SchemaExport tool.)

<property name="serialNumber" unique="true" type="string" column="SERIAL_NUMBER"/>
Then the mapping for OrderItem might use:

<many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>
This is certainly not encouraged, however.