HTTP auth = new HTTP();
    HTTPRequest r = new HTTPRequest();
    r.setEndpoint('https://sb-abbvie-ae.veevavault.com/api/v17.1/auth?');
    String jsonBody ='username=shardul.deshmukh@abbviesbx.com&password=Vault@2017';
    r.setBody(jsonBody);
    r.setMethod('POST');
    r.setHeader('Content-Type','application/x-www-form-urlencoded');
    HTTPResponse authresp=new HttpResponse();
    authresp = auth.send(r);

system.debug('authresp@'+authresp.getBody());

authresp@{"responseStatus":"SUCCESS","sessionId":"880817F7FCED781F4687214AD6201B4466C3CD61E435B987C802AC2DB46D92781308B080996C31FF1E6A35AC","userId":XXXX,"vaultIds":[{"id":XXXX,"name":"XXXXX","url":"https://XXXXXX-XXX.com/api"}],"vaultId":XXXX}