On Windows, try to use the Java bundled with VASSAL.

vasl_templates/webapp/static/help/index.html

@@ -139,7 +139,8 @@ Configuring the program
     <tr> <td class="key"> VASL module: </td> <td class="val"> <nobr>C:\bin\vasl\vasl-6.6.1.vmod</nobr>
     <tr> <td class="key"> VASL extensions: </td> <td class="val"> <nobr>C:\bin\vasl\extensions\</nobr>
     <tr> <td class="key"> VASL boards: </td> <td class="val"> <nobr>C:\bin\vasl\boards\</nobr>
-    <tr> <td class="key"> Java: </td> <td class="val"> <nobr>C:\Program Files\VASSAL-3.4.6\jre\bin\java.exe</nobr>
+    <tr> <td class="key" valign="top"> Java: </td> <td class="val"> <nobr>C:\Program Files\VASSAL-3.4.6\jre\bin\java.exe</nobr>
+        <div class="hint"> If you're on Windows, you can leave this field blank, <br> and the version of Java that comes with VASSAL will be used. </div>
     <tr> <td class="key"> Web driver: </td> <td class="val"> <nobr>C:\bin\geckodriver.exe</nobr>
 </table>
 For the webdriver, download and unpack one of the following:

vasl_templates/webapp/utils.py

@@ -235,6 +235,10 @@ def parse_int( val, default=None ):
     except (ValueError, TypeError):
         return default
 
+def is_windows():
+    """Check if we're running on Windows."""
+    return os.name == "nt"
+
 # ---------------------------------------------------------------------
 
 def compare_version_strings( lhs, rhs  ):

vasl_templates/webapp/vassal.py

@@ -17,7 +17,7 @@ from flask import request, jsonify
 
 from vasl_templates.webapp import app, globvars
 from vasl_templates.webapp.config.constants import BASE_DIR, IS_FROZEN
-from vasl_templates.webapp.utils import TempFile, SimpleError, compare_version_strings
+from vasl_templates.webapp.utils import TempFile, SimpleError, compare_version_strings, is_windows
 from vasl_templates.webapp.webdriver import WebDriver
 from vasl_templates.webapp.vasl_mod import get_reverse_remapped_gpid
 
@@ -393,6 +393,11 @@ class VassalShim:
             if compare_version_strings( mo.group(), "3.3.0" ) < 0:
                 # we're using a legacy version of VASSAL - use Java 8
                 java_path = java8_path
+        if not java_path and is_windows():
+            # we're on Windows - try to use the Java that is now bundled with VASSAL
+            fname = os.path.join( self._get_vassal_dir(), "jre/bin/java.exe" )
+            if os.path.isfile( fname ):
+                java_path = fname
         if not java_path:
             java_path = "java" # nb: this must be in the PATH