diff --git a/pex/third_party/__init__.py b/pex/third_party/__init__.py
index 39e7df9f4..4403b0ab2 100644
--- a/pex/third_party/__init__.py
+++ b/pex/third_party/__init__.py
@@ -177,7 +177,7 @@ def _vendored_path_items():
             for spec in vendor.iter_vendor_specs(
                 # N.B.: The VendorImporter should only see the versions of vendored projects that
                 # support the current Python interpreter.
-                filter_requires_python=True
+                filter_requires_python=sys.version_info[:2]
             )
         )
 
@@ -280,7 +280,7 @@ def expose(
 
         def iter_available():
             yield "pex", root  # The pex distribution itself is trivially available to expose.
-            for spec in vendor.iter_vendor_specs(interpreter=interpreter):
+            for spec in vendor.iter_vendor_specs(filter_requires_python=interpreter):
                 yield spec.key, spec.relpath
 
         path_by_key = OrderedDict(
@@ -473,7 +473,7 @@ def isolated(interpreter=None):
         # PEX_ROOT or built PEXs.
         vendor_lockfiles = tuple(
             os.path.join(os.path.relpath(vendor_spec.relpath, module), "constraints.txt")
-            for vendor_spec in vendor.iter_vendor_specs(interpreter=interpreter)
+            for vendor_spec in vendor.iter_vendor_specs(filter_requires_python=interpreter)
         )
 
         pex_zip_paths = None  # type: Optional[Tuple[str, str]]
diff --git a/pex/vendor/__init__.py b/pex/vendor/__init__.py
index 244d30b6d..1eb8a9e07 100644
--- a/pex/vendor/__init__.py
+++ b/pex/vendor/__init__.py
@@ -14,7 +14,7 @@
 from pex.typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
-    from typing import Iterable, Iterator, Optional, Sequence, Set, Text, Tuple
+    from typing import Iterable, Iterator, Optional, Sequence, Set, Text, Tuple, Union
 
     from pex.interpreter import PythonInterpreter
 
@@ -158,15 +158,22 @@ def create_packages(self):
             touch(os.path.join(self.ROOT, *relpath))
 
 
-def iter_vendor_specs(
-    filter_requires_python=False,  # type: bool
-    interpreter=None,  # type: Optional[PythonInterpreter]
-):
-    # type: (...) -> Iterator[VendorSpec]
+def iter_vendor_specs(filter_requires_python=None):
+    # type: (Optional[Union[Tuple[int, int], PythonInterpreter]]) -> Iterator[VendorSpec]
     """Iterate specifications for code vendored by pex.
 
+    :param filter_requires_python: An optional interpreter (or its major and minor version) to
+                                   tailor the vendor specs to.
     :return: An iterator over specs of all vendored code.
     """
+    python_major_minor = None  # type: Optional[Tuple[int, int]]
+    if filter_requires_python:
+        python_major_minor = (
+            filter_requires_python
+            if isinstance(filter_requires_python, tuple)
+            else filter_requires_python.version[:2]
+        )
+
     # We use this for a better @dataclass that is also Python2.7 and PyPy compatible.
     # N.B.: The `[testenv:typecheck]` section in `tox.ini` should have its deps list updated to
     # reflect this attrs version.
@@ -180,22 +187,18 @@ def iter_vendor_specs(
 
     # We use this via pex.third_party at runtime to check for compatible wheel tags and at build
     # time to implement resolving distributions from a PEX repository.
-    # TODO(John Sirois): XXX: filter_requires_python = False + interpreter non None is a bit
-    #  contradictory.
-    filter_requires_python = filter_requires_python or interpreter is not None
-    python_major_minor = interpreter.version[:2] if interpreter else sys.version_info[:2]
-    if not filter_requires_python or python_major_minor < (3, 6):
+    if not python_major_minor or python_major_minor < (3, 6):
         # N.B.: The pyparsing constraint is needed for 2.7 support.
         yield VendorSpec.pinned(
             "packaging", "20.9", import_path="packaging_20_9", constraints=("pyparsing<3",)
         )
-    if not filter_requires_python or python_major_minor == (3, 6):
+    if not python_major_minor or python_major_minor == (3, 6):
         # N.B.: The pyparsing constraint is needed because our import re-writer (RedBaron) chokes on
         # newer versions.
         yield VendorSpec.pinned(
             "packaging", "21.3", import_path="packaging_21_3", constraints=("pyparsing<3",)
         )
-    if not filter_requires_python or python_major_minor >= (3, 7):
+    if not python_major_minor or python_major_minor >= (3, 7):
         yield VendorSpec.pinned("packaging", "23.1", import_path="packaging_23_1")
 
     # We use toml to read pyproject.toml when building sdists from local source projects.