Hello.
It seems to me that it is not need to concatenate subdomains and url.
gobuster vhost -w subdomains.txt -u http://site.company.org/
It concatenates each subdomain with site.company.org = subdomain.site.company.org. Then I expected subdomain.company.org
Actually it may be usefull only in one case when I need to discover subdomains in main domain:
gobuster vhost -w subdomains.txt -u http://company.org/
But very offten webservers may server many sites (subdomains) from variuos domain like a subdomain.company.org or subdomain.old-company.org.
Also I can't use it if site hasn't active domain and has just IP:
gobuster vhost -w subdomains.txt -u http://1.2.3.4/
In result it will be subdomain.1.2.3.4 it is stuff.
Hovewer this web-server also could be contains old sites which names may be obtained from passive-dns.

I think that gobuster vhost don't need contatenate sub + domain.com. Each entry from subdomains.txt must be inserted in host header without any concatenation like a FQDN.

Thank you.