Vermouths range a lot in flavour, some work better in some drinks than others. Antica Formula is the common expensive one, and makes a great Manhattan. IMO it's too sweet in Negroni and variations though. Actually my favourite in a Negroni may be Martini Rosso, which is the cheapest one you can get
So vermouth flavour is really important in a Manhattan where it's just 2 parts and bitters!
Also make sure it's fresh. Once you open a bottle, it will start to go bad. Keep it in the fridge and it can last maybe a month. Out of a fridge, it'll be rotten in 3-5 days
Also that combo may not just be great. Really old school Manhattan recipes used bourbon (like pre-prohibition or even earlier) but they quickly switched to Rye and IIRC it's pretty much been rye ever since. The spicy rye may just mix better with the sweet vermouth and bitters to round them out.
Bourbon + Campari + Vermouth though
