The configuration files inside the vmnet8 and vmnet1 directories will get re-generated automatically using the settings in the /Library/Preferences/VMware Fusion/networking file. There is a command-line utility to modify the file, although it's not very well documented. Here are the commands that should be able to change the DHCP default and maximum lease times for vmnet8:
sudo /Applications/VMware\ Fusion.app/Contents/Library/vmnet-cfgcli setdhcpparam vmnet8 defleasetime <seconds>
sudo /Applications/VMware\ Fusion.app/Contents/Library/vmnet-cfgcli setdhcpparam vmnet8 maxleasetime <seconds>
Similarly, you can read the configuration parameters:
sudo /Applications/VMware\ Fusion.app/Contents/Library/vmnet-cfgcli getdhcpparam vmnet8 defleasetime
sudo /Applications/VMware\ Fusion.app/Contents/Library/vmnet-cfgcli getdhcpparam vmnet8 maxleasetime
The new configuration should take effect the next time Fusion is launched, or you can also manually apply it:
sudo /Applications/VMware\ Fusion.app/Contents/Library/vmnet-cli --configure
Afterwards, check the contents of vmnet8/dhcpd.conf to make sure the values were set accordingly.
However, I'm not sure that the issue you originally reported is happening because of a DHCP lease expiration - the log messages seem to suggest that the bridged interface (not NAT) was disconnected. Could you post the messages you are still seeing in the log with Fusion 5.0.3?