The problem may be (it was in my case) that there are firewall rules preventing NAT firewall rules for the virbr0 network device created by Vagrant via libvirt, which may look like the following (excerpt of iptables -L -n):
Chain FORWARD (policy ACCEPT) target prot opt source destination ACCEPT all -- 0.0.0.0/0 192.168.121.0/24 ctstate RELATED,ESTABLISHED ACCEPT all -- 192.168.121.0/24 0.0.0.0/0
A possible fix is to disable those restrictive rules (for example, clear all iptables rules before starting the Vagrant machine).