I always think that the accessories can really lift a costume and make it your own. The choices are limitless both with jewellery (Regency and Victorian gentlemen would have had pocket watches, fobs and seals, signet rings, tie pins etc) and maybe a cane or some weaponry. (Or both). The important thing is to have fun and not worry too much about historical accuracy - we're reinventing the past not recreating it

.
I think it helps to latch on to a persona of sorts. Build a bit of a fantasy behind the costume. Then you can think of all sorts of pretexts as to what accesories you add to the costume, as well as any props.
In fact, you are not looking at recreating historical attire, as much as you are looking to adapt the attire to a fictional background. Have fun with it.
For example look at my costume in the Playing Dressup thread. This is one of two "personas" I have; Lt. Gen. Julin W. Bahlmann. These personas are actually tied to a fiction project I have in mind, "The Valkyrie and the Eagle."
I give some background on the costume for Bahlmann here (photos):
http://brassgoggles.co.uk/forum/index.php/topic,35457.msg981130.html#msg981130And then I expound on the story background of the costume in the "Rave about your steampunk characters/personas" thread. Since these are the main characters of a novel, you have to tie Bahlmann's origin to that of my first persona (and namesake), Admiral J. Wilhelm:
Adm. Wilhelm's origin:
http://brassgoggles.co.uk/forum/index.php/topic,49108.msg983624.html#msg983624Lt. Gen. Bahlmann's origin:
http://brassgoggles.co.uk/forum/index.php/topic,4918.msg983625.html#msg983625I still have to come up for a uniform for "Old Walrus" Wilhelm. It's a very expensive costume, so I'm deferring for later, but I fully developed the other character's costume, and I've been building on it for years!
You don't have to go this far, of course. I am writing a novel. So you don't really have to do any of this. But for me, writing about stuff around my characters, and doing research has helped my imagination, to come up with the costume itself. After that, it becomes like any other hobby, you just build on it...
~ ~ ~
Also, should I get a vest?
Probably. Maybe... I think a vest is a good idea. I won't call it a *must*, because like Ms. Courcelle has stated this is not recreation. Again, I point to my post on the link above... I certainly don't wear a vest

However, in the historical sense, shirts were considered to be stricitly "underwear" prior to the 1810's or so. People usually wore something on top of a shirt, even if it was just covered partially.
A brief history of the shirt and blouse as a stand alone garment...
The first instance of shirts being worn without a vest or coat were among the working class in America, in the 1810's (in the form of thick plaid shirts) and later, I believe among the military, in the 1860s.
During the Italian Reunification, shirts worn alone emerged with Garibaldi's insurgent troops known as the "red shirts." Thereafter the "Garibaldis" as the shirts were known, became the inspiration for women's blouses (a light, loose garment top, separate from a dress, of which we were (and are) all very familiar with in the 20th. and 21st C.
In the meantime,the working man's plaid shirt attained more patterns and solid colours, and then became an acceptable item to wear alone in the 20th. C, say by the Jazz age, and again probably in America first.
The other instance of a "shirt alone" also came among the American military, in the form of the "Fatigue Blouse" after the American Civil War. Basically a thick shirt resembling a light coat, the Fatigue Blouse was meant to replace the service wool coats of the US Army. This was an absolutely necessary change, as the Civil War was proven to be impossible to wage in the heat of the American Southwest (battles for the Arizona and New Mexico territories, ca. 1863 were settled in less than 12 months. The soldiers couldn't fight!). The idea, being that by the period of the Indian wars, the fatigue blouse allowed the service man to wear or take off as many layers of undergarments as needed, while still wearing a formal attire on the outside. In time, the fatigue blouse attained drab solid colours, became the field attire (totally separate from a "service coat" to be worn in the office), and eventually became a camouflaged garment during the 2nd. half of 20th. C. as we all now understand the term "Fatigues" in the 21st. C.