I wanna try to do a vinegar etch to get a deep black finish. My question is do I need to get it to a mirror finish, with no scratches, or just sand it to exposed steel? Or will the vinegar remove the forge scale by itself if I just leave it in longer? Thanks in advance!

I think you have a terminology problem here. Etch isn't quite the same thing as patination. If all you're after is a strong black use a phosphorous acid based product and Parkerize it. It will turn most iron alloys FLAT BLACK. The last I bought was called, "Ospho," but it's been 30 years and it may not even be around now. Naval Jelly will turn steel parkerized black if you don't neutralize and rinse it off. There are better products of course or you could look in gun magazines. 

Yes, you want to remove scale and oil before etching or other patination. Mirror polish isn't necessary but make sure it's smooth and the desired surface. Wire brushed or bead blasted are good too.

What are you making? We love pictures you know.

Frosty The Lucky. 

vinegar can be used to bring out hamones and pantern welds. same technique japanese would use to polish blades except (correct me if im wrong) i think they use lemon juice traditionally. 

acid polid rinse repeat.  ive done it on a san mai and got some cool results i took quite a bit of effort as you dont want the vinager to sit too long or it will leave a rust color

 

Yes; you can use a vinegar soak to remove scale.  Usually I do it overnight with full strength "household vinegar" and then wash off the black crud under running water and oil immediately to avoid flash rusting!  Have the item completely immersed as liquid/air transitions can etch into a surface.

Thomas, I use exactly the same technique as you described, but if I plan on painting or otherwise finishing it, I have to skip the oiling step and go directly to the finishing process immediately!  The oil will prohibit most good paint or other finish bonding.

<blush> I thought everybody knew that<blush-off> 

In the painting case one of the phosphoric pre-painting rinses after the vinegar etch and water rinse might be indicated.

Thomas, you're making me blush........

I thought it was good practice to rinse with baking soda solution to prevent rusting issues before applying oil after an acid etch. Is that overkill?

No.