Modded clients can connect to vanilla servers again. Modded to modded seems to need some work still...

proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftConnection.java

@@ -47,6 +47,7 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
     private MinecraftConnectionAssociation association;
     private boolean isLegacyForge;
     private final VelocityServer server;
+    private boolean canSendLegacyFMLResetPacket = false;
 
     public MinecraftConnection(Channel channel, VelocityServer server) {
         this.channel = channel;
@@ -231,4 +232,12 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
     public void setLegacyForge(boolean isForge) {
         this.isLegacyForge = isForge;
     }
+
+    public boolean canSendLegacyFMLResetPacket() {
+        return canSendLegacyFMLResetPacket;
+    }
+
+    public void setCanSendLegacyFMLResetPacket(boolean canSendLegacyFMLResetPacket) {
+        this.canSendLegacyFMLResetPacket = isLegacyForge && canSendLegacyFMLResetPacket;
+    }
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -47,7 +47,6 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
             connection.getPlayer().handleConnectionException(connection.getServerInfo(), original);
         } else if (packet instanceof JoinGame) {
             playerHandler.handleBackendJoinGame((JoinGame) packet);
-            connection.setHasCompletedJoin(true);
         } else if (packet instanceof BossBar) {
             BossBar bossBar = (BossBar) packet;
             switch (bossBar.getAction()) {
@@ -74,7 +73,8 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
                 if (!connection.isModded()) {
                     connection.setModded(true);
 
-                    // We must always reset the handshake before a modded connection is established.
+                    // We must always reset the handshake before a modded connection is established if
+                    // we haven't done so already.
                     connection.getPlayer().sendLegacyForgeHandshakeResetPacket();
                 }
 
@@ -113,13 +113,14 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
     private boolean canForwardPluginMessage(PluginMessage message) {
         ClientPlaySessionHandler playerHandler =
                 (ClientPlaySessionHandler) connection.getPlayer().getConnection().getSessionHandler();
-        boolean isMCMessage;
+        boolean isMCOrFMLMessage;
         if (connection.getMinecraftConnection().getProtocolVersion() <= ProtocolConstants.MINECRAFT_1_12_2) {
-            isMCMessage = message.getChannel().startsWith("MC|");
+            String channel = message.getChannel();
+            isMCOrFMLMessage = channel.startsWith("MC|") || channel.startsWith(VelocityConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL);
         } else {
-            isMCMessage = message.getChannel().startsWith("minecraft:");
+            isMCOrFMLMessage = message.getChannel().startsWith("minecraft:");
         }
-        return isMCMessage || playerHandler.getClientPluginMsgChannels().contains(message.getChannel()) ||
+        return isMCOrFMLMessage || playerHandler.getClientPluginMsgChannels().contains(message.getChannel()) ||
                 server.getChannelRegistrar().registered(message.getChannel());
     }
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -197,6 +197,8 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
         // Flush everything
         player.getConnection().flush();
         player.getConnectedServer().getMinecraftConnection().flush();
+        player.getConnectedServer().setHasCompletedJoin(true);
+        player.getConnection().setCanSendLegacyFMLResetPacket(true);
     }
 
     public List<UUID> getServerBossBars() {

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -296,10 +296,13 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     }
 
     public void sendLegacyForgeHandshakeResetPacket() {
+        if (connection.canSendLegacyFMLResetPacket()) {
             PluginMessage resetPacket = new PluginMessage();
             resetPacket.setChannel(VelocityConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL);
             resetPacket.setData(VelocityConstants.FORGE_LEGACY_HANDSHAKE_RESET_DATA);
             connection.write(resetPacket);
+            connection.setCanSendLegacyFMLResetPacket(false);
+        }
     }
 
     public void close(TextComponent reason) {