I have a site in heroku with a front end built in vue.js and I just bought a domain in godaddy.
I can access the site via the new domain, however when I access the site via the new domain, there is something wrong with the css because the site has the same layout(on my laptop and phone). However, if i access it via the ******.herokuapp.com it is responsive. How can I make the site responsive using the new domain ?
It sounds like you may be using Masking on the domain, which should be removed.
Generally for Heroku you would point the domain to your hosting account using a CNAME. If it's for the main site you would use www, then regular forwarding (301 without masking) the bare yourdomain.com to www.yourdomain.com