Mike - Long discussion for several easy techniques. I hope this helps:
When you enter an address in the to field of a message, the auto fill completes the address, but if you wait before clicking to enter the address, a list pops up showing all the addresses for that person listed in your Address Book. You can drag down through the list and click one to complete the address entry in the "To" field with the selected address. If you want to send the message to other email addresses for that person, you will have to start entering the person's name again to have the opportunity to choose another address from the pop up list.
If you want to see the addresses associated with names already entered into the "To" field you can hover the cursor over each name to see a box with the entire name/address. Or click and hold on each entry - it will expand to show name and email address until you release. Entries with multiple addresses will show as a list with a check mark indicating which address is active.
I guess you could also make an address group for a multi addressed contact, and enter a separate contact in the group for each email address of that person. Then when you enter the group in your To field, each address in the group will get the email.
I don't think separate vCards will help for this, because you will still have to enter each contact in your "To" field whether they are on separate cards or are multiple address entries in the same card.
-B