Code P0400= EGR system Malfunction
Yes they could be related as the VSS affects the vaccuum to the egr valve.( The vss tells the pcm to open the egr valve as it moves thus affecting the vacuum to the egr valve, meaning that te front wheels need to be moving in order to check correct vacuum to egr valve at higher rpm.)) I would check the vss connector on the transaxle for looseness and corrided connector, if that not the problem I would check the gear on the vss for wear. These VSS'S are usually a bear to get out, many people have broken them trying to remove, make sure to use pliers or vise grip on the metal base of the vss.
Another thing to do is to clean the MAF Sensor as this will greatly affect mileage and performance of engine, or TPS (Throttle Position Sensor) could be flaking out as it is a wear item.
report back with results.