Use slot

src/components/ModalWindow.vue

@@ -1,13 +1,7 @@
 <template>
 	<div class='modal_window__overlay' :class='{"modal_window--show": internalShowModal}' @click.self='hideModal'>
 		<div class='modal_window' :class='{"modal_window--show": internalShowModal}'>
-			Placeholder content<br/>
-			Placeholder content<br/>
-			Placeholder content<br/>
-			Placeholder content<br/>
-			Placeholder content<br/>
-			Placeholder content<br/>
-			Placeholder content<br/>
+			<slot></slot>
 		</div>
 	</div>
 </template>

src/components/routes/Index.vue

@@ -1,6 +1,10 @@
 <template>
 	<div class='index'>
-		<modal-window :showModal='showModal'></modal-window>
+		<modal-window :showModal='showModal'>
+			<slot>
+				
+			</slot>
+		</modal-window>
 
 
 		<div class='thread_sorting'>