New try for the port...

conf/vite.config.js

@@ -0,0 +1,16 @@
+import { sveltekit } from '@sveltejs/kit/vite';
+import path from 'path';
+
+/** @type {import('vite').UserConfig} */
+const config = {
+	resolve: {
+		alias: {
+			'xmlhttprequest-ssl': './node_modules/engine.io-client/lib/xmlhttprequest.js',
+			'@api': path.resolve('./src/api')
+		}
+	},
+	plugins: [sveltekit()],
+	server: {
+    	port: __PORT__,
+  	},
+};

scripts/install

@@ -162,8 +162,8 @@ popd
 mkdir -p "$final_path/web/"
 chown -R $app:$app $final_path/build/output/usr/src/app
 pushd $final_path/build/output/usr/src/app
-	sed -i 's/3000/'$port'/' package.json
-	sed -i 's/vite build/vite build --port '$port'/' package.json
+	rm vite.config.js
+	ynh_add_config --template="../conf/vite.config.js" --destination="$final_path/build/output/usr/src/app/vite.config.js"
 	ynh_exec_warn_less sudo -u $app env $ynh_node_load_PATH npm ci
 	ynh_exec_warn_less sudo -u $app env $ynh_node_load_PATH npm run build
 popd