You can have 2 different subnets in one VLAN without problems. Simple add the second IP address in VLAN1 interface config (depend on version of IOS SW you will maybe need add 'secondary' keyword).
Two VLANs will divide this two subnets on the 2nd layer of ISO/OSI models, in the other view - you will have two bridges (every VLAN acts as a bridge) and Cisco will routing between it. This is an advance, when you want to divide access to some network resources between subnets and you want have firewall between those subnets. As a result, you will have to add every port of Cisco box only to one VLAN (even 1 or 250, but not both!) and then on every port you will have only one subnet. There is a way to move both subnets to another switch via 'trunk' interfaces, but for normal PC's you have to have switchports in only one VLAN.