Do vendors increase their costs in peak season?

Most hairdressers I found charged the same rate all year. Photographers didn't tend to offer a cheaper rate out of season, but they were sometimes willing to provide reduced coverage (e.g. half a day, not a whole day) for midweek or out of season weddings - something to consider if you're not fussed about having every little detail recorded. With florists, it very much depends on the kind of flowers you want, but you will have a wider availability in July, especially of local flowers, which might keep your costs down.

One thing that will make a difference - if you are honeymooning straight after the wedding, then the prices are likely to shoot up once you get into school holidays, so March (avoiding Easter) might be the best bet for that.

With photographers at least Its the other way around really as summer is the normal cost but are willing to shoot for less in the winter.

I have always thought that winter weddings should cost more because they are much more challenging to photograph. So if you do choose March then make sure that you potential photographer can show you plenty of example weddings that they have shot in winter.