Here's what ended up working for me:
I viewed the plugins on Ionic's site: https://ionicframework.com/docs/native/in-app-purchase (in my case) and then for the install I ran npm install cordova-plugin-inapppurchase and then ran npm install @ionic-native/in-app-purchase. Then in my Vue JS file I imported the plugin using import { InAppPurchase } from '@ionic-native/in-app-purchase/ngx';
The next part is what tripped me up. On the next line I had to create a new variable to access the referenced plugin. So the next line of code is const iap = new InAppPurchase();
Then you can access the iap variable and use all the documented methods from within your Vue app. 
Keep in mind that any testing of Cordova plugins will need to happen on your device.
Hope this helps anyone who was struggling like me!