Hi Mickey, welcome to BF. Fitting a bike is more than just your height an inseam measurement, its best to find a local bike Shop (LBS in BF speak) that will work with you and listen as well as sell to get you the right bike and to fit it to you, properly. I'm 6'-1" also but I have short legs so I am on a 56cm frame, but many others in our height are on 58cm-60cm frames depending on their build, fitness and fitness. Check in to the Road Cycling Forum and read some of the threads about fit and frames, just take them with a grain of salt and don't let them seem serious with the kidding, they are a knowledgeable bunch, but don't say I told you that. Even if you are limited by your budget an LBS can help you out and there is always eBay and Craig's List for used bikes. Best of luck.
If you are in the UK, and I assume by the web site for the Vertigo bike that you are, try Ribble and Wiggle to start, they both offer bikes like Tesco does. That is a low end bike, its components will be alright for a short while but it may not hold up well. Forget Craig's List in the UK, sorry about that one, my bad. and yes a 21" frame is pretty small for your size. Try to avoid department store bikes, they aren't assembled well and have a lot of corners cut as far as quality. I subscribe to Cycling Plus, from the UK, buy an issue of it or another road bike magazine and read up, them go to a few bike shops and maybe a local club that will work with you. Best of luck, you'll find the right bike.