diff --git a/renderdoc/driver/vulkan/vk_resources.h b/renderdoc/driver/vulkan/vk_resources.h
index c4fd994fa..ed85533c6 100644
--- a/renderdoc/driver/vulkan/vk_resources.h
+++ b/renderdoc/driver/vulkan/vk_resources.h
@@ -686,9 +686,11 @@ struct MemMapState
 {
 	MemMapState()
 		: mapOffset(0), mapSize(0)
-		, mapFlushed(false), mapCoherent(false), mappedPtr(NULL), refData(NULL)
+		, needRefData(false), mapFlushed(false), mapCoherent(false)
+		, mappedPtr(NULL), refData(NULL)
 	{ }
 	VkDeviceSize mapOffset, mapSize;
+	bool needRefData;
 	bool mapFlushed;
 	bool mapCoherent;
 	byte *mappedPtr;
diff --git a/renderdoc/driver/vulkan/wrappers/vk_queue_funcs.cpp b/renderdoc/driver/vulkan/wrappers/vk_queue_funcs.cpp
index 06c3b7e29..4571b3498 100644
--- a/renderdoc/driver/vulkan/wrappers/vk_queue_funcs.cpp
+++ b/renderdoc/driver/vulkan/wrappers/vk_queue_funcs.cpp
@@ -593,16 +593,26 @@ VkResult WrappedVulkan::vkQueueSubmit(
 							(VkDeviceMemory)(uint64_t)record->Resource,
 							state.mapOffset+diffStart, diffEnd-diffStart
 						};
+						// this causes the call within to allocate state.refData and copy what was
+						// serialised into it. We want to copy *precisely* the serialised data,
+						// otherwise there is a gap in time between serialising out a snapshot of
+						// the buffer and whenever we then copy into the ref data, e.g. below.
+						// during this time, data could be written to the buffer and it won't have
+						// been caught in the serialised snapshot, and if it doesn't change then
+						// it *also* won't be caught in any future FindDiffRange() calls.
+						//
+						// Note: it's still possible that data is being written to by the
+						// application while it's being serialised out in the snapshot below. That
+						// is OK, since the application is responsible for ensuring it's not writing
+						// data that would be needed by the GPU in this submit. As long as the
+						// refdata we use for future use is identical to what was serialised, we
+						// shouldn't miss anything
+						state.needRefData = true;
 						vkFlushMappedMemoryRanges(dev, 1, &range);
 						state.mapFlushed = false;
 					}
 
 					GetResourceManager()->MarkPendingDirty(record->GetResourceID());
-
-					// allocate ref data so we can compare next time to minimise serialised data
-					if(state.refData == NULL)
-						state.refData = Serialiser::AllocAlignedBuffer((size_t)state.mapSize, 64);
-					memcpy(state.refData, state.mappedPtr, (size_t)state.mapSize);
 				}
 				else
 				{
diff --git a/renderdoc/driver/vulkan/wrappers/vk_resource_funcs.cpp b/renderdoc/driver/vulkan/wrappers/vk_resource_funcs.cpp
index e22fbb7d8..a76a22464 100644
--- a/renderdoc/driver/vulkan/wrappers/vk_resource_funcs.cpp
+++ b/renderdoc/driver/vulkan/wrappers/vk_resource_funcs.cpp
@@ -487,6 +487,25 @@ bool WrappedVulkan::Serialise_vkFlushMappedMemoryRanges(
 	SERIALISE_ELEMENT(uint64_t, memSize, pMemRanges->size);
 	SERIALISE_ELEMENT_BUF(byte*, data, state->mappedPtr + (size_t)memOffset, (size_t)memSize);
 
+	// if we need to save off this serialised buffer as reference for future comparison,
+	// do so now. See the call to vkFlushMappedMemoryRanges in WrappedVulkan::vkQueueSubmit()
+	if(m_State >= WRITING && state->needRefData && !state->refData)
+	{
+		// if we're in this case, the range should be for the whole memory region.
+		RDCASSERT(memOffset == 0 && memSize == state->mapSize);
+
+		// it's no longer safe to use state->mappedPtr, we need to save *precisely* what
+		// was serialised. We do this by copying out of the serialiser since we know this
+		// memory is not changing
+		size_t offs = size_t(localSerialiser->GetOffset() - memSize);
+
+		byte *serialisedData = localSerialiser->GetRawPtr(offs);
+
+		// allocate ref data so we can compare next time to minimise serialised data
+		state->refData = Serialiser::AllocAlignedBuffer((size_t)state->mapSize);
+		memcpy(state->refData, serialisedData, (size_t)state->mapSize);
+	}
+
 	if(m_State < WRITING)
 	{
 		device = GetResourceManager()->GetLiveHandle<VkDevice>(devId);