A proxy server may ask a client to be authenticated before using the relay. The authentication method can be Basic or NTLM. To find out which method is used by the browser, you can use chrome to open www.google.com and inspect the request headers.
If the headers contain a line like "Proxy-Authorization:NTLM xxxx", it means the proxy uses NTLM to authenticate the user. For the command line applications such as maven or vagrant, even eclipse, they cannot make use this kind proxy server directly, because they don't know how to generate the NTLM authentication hash.
Open a command window and type
set http_proxy=http://proxy.mycompany.com:8080
curl -v http://www.google.com
You'll see that the authentication fails.
To solve the problem you need to setup a authentication proxy between the company proxy and the client. Download Cntlm from http://cntlm.sourceforge.net/ and unzip the file, then open a command window
cntlm.exe -I -M http://www.google.com -u USER@DOMAIN proxy.mycompany.com:8080
The program will prompt you to enter your windows user password and generate the NTLMv2 hash. Now you open the file cntlm.ini in the same folder as cntlm.exe, put the generate hash to the PassNTLMv2.
cntlm.exec -c cntlm.ini
set http_proxy=http://localhost:3128
set https_proxy=https://localhost:3128
curl -v http://www.google.com