how can bundle sql express edition in to my vb app. so that sqel express edition install when i install the app
You can use an installation authoring tool for this purpose which will make your task easier. I would suggest you to try InstallAware which allows one click deployment of runtimes like SQL Server Express and such. This installation tool will bundle up the pre-requisites in a single click. Also, you can take advantage of its superior compression capability in which your pre-requisites will be compressed to a fraction of their original size. If your selected pre-requisite has further dependencies, InstallAware includes them for you as well - automatically.
You can even check for a named instance and install only if that named instance is not found - true to the spirit of pre-requisites. They will only be downloaded if they are not already available on the target system.