Chaplin Harris
1. Is it inappropriate to not wear the vest?

Answer to this is no it is not inappropriate, however do wear a dress belt with it.

2. If I can get by without the vest, would my tartan tie look moronic?

I think you could away with it, I have worn a regular tie with my P.C.

3. If NO to the two above, I don't have a bow tie to use, what advice about what to wear with it?

I did recently see a tuxedo shirt/bowtie combo at walmart and you might be able to find something like that.

Also a few days ago I went into my local tartan shop and picked up two Argylle jackets that were on consignment, total cost for two jackets $120.00
One is a Harris Tweed, that was on sale for the paltry sum of $40.00, perhaps you could find something like that.

As for your somewhat laidback congregation, I would say wear the P.C. proudly regardless of what they choose to wear to church. You will of course be in front so I really don't think that should be an issue.