try this code..
function ShowModal() {
ExecuteOrDelayUntilScriptLoaded(function () {
var options = {
url: 'servername/webpart.aspx',
tite: 'Add Vendor',
width: 800,
height: 475,
allowMaximize: true,
showClose: true,
dialogReturnValueCallback: scallback
};
SP.UI.ModalDialog.showModalDialog(options);
}, 'sp.js');
}
function scallback(dialogResult, returnValue) {
if (dialogResult == SP.UI.DialogResult.OK) {
SP.UI.ModalDialog.RefreshPage(SP.UI.DialogResult.OK);
}
Source : SP.UI.ModalDialog.RefreshPage from the dialog itself?SP.UI.ModalDialog.RefreshPage from the dialog itself?