diff --git a/src/m_default/_default.c b/src/m_default/_default.c
index f0caf01..06928a5 100644
--- a/src/m_default/_default.c
+++ b/src/m_default/_default.c
@@ -73,6 +73,7 @@ void stdio_usb_set_itf_num(int itf); // TODO: move to a header!
 
 static void enter_cb(void) {
     stdio_usb_set_itf_num(CDC_N_STDIO);
+    vnd_cfg_set_itf_num(VND_N_CFG);
 
     // TODO: CMSISDAP?
 #ifdef DBOARD_HAS_I2C
diff --git a/src/m_default/cdc_serprog.c b/src/m_default/cdc_serprog.c
index 490abb8..52fc195 100644
--- a/src/m_default/cdc_serprog.c
+++ b/src/m_default/cdc_serprog.c
@@ -76,9 +76,9 @@ static uint8_t read_byte_cdc(void) {
 }
 
 static uint32_t nresp = 0;
-static void handle_cmd(uint8_t cmd, int ud, uint8_t (*read_byte)(void),
-        void (*writepkt)(int ud, const uint8_t* buf, uint32_t len),
-        void (*flushpkt)(int ud),
+static void handle_cmd(uint8_t cmd, uint8_t ud, uint8_t (*read_byte)(void),
+        uint32_t (*writepkt)(uint8_t ud, const void* buf, uint32_t len),
+        uint32_t (*flushpkt)(uint8_t ud),
         void (*writehdr)(enum cfg_resp stat, uint32_t len, const void* data)) {
     nresp = 0;
 
@@ -304,15 +304,19 @@ void cdc_serprog_task(void) {
 }
 
 
-static void vnd_writepkt(int ud, const uint8_t* buf, uint32_t len) {
+static uint32_t vnd_writepkt(uint8_t ud, const void* buf, uint32_t len) {
     (void)ud;
 
-    for (size_t i = 0; i < len; ++i) vnd_cfg_write_byte(buf[i]);
+    for (size_t i = 0; i < len; ++i) vnd_cfg_write_byte(((const uint8_t*)buf)[i]);
+
+    return len;
 }
-static void vnd_flushpkt(int ud) {
+static uint32_t vnd_flushpkt(uint8_t ud) {
     (void)ud;
 
     vnd_cfg_write_flush();
+
+    return 0;
 }
 void sp_spi_bulk_cmd(void) {
     uint8_t cmd = read_byte_cdc();
diff --git a/src/vnd_cfg.c b/src/vnd_cfg.c
index 4346c69..d7065dc 100644
--- a/src/vnd_cfg.c
+++ b/src/vnd_cfg.c
@@ -16,12 +16,18 @@ static uint8_t tx_buf[CFG_TUD_VENDOR_TX_BUFSIZE];
 
 static uint32_t rxavail, rxpos, txpos;
 
-static const int VND_N_CFG;
+static int VND_N_CFG = 0;
 
 void vnd_cfg_init(void) {
     rxavail = 0;
     rxpos   = 0;
     txpos   = 0;
+
+    VND_N_CFG = 0;
+}
+
+void vnd_cfg_set_itf_num(int itf) {
+    VND_N_CFG = itf;
 }
 
 uint8_t vnd_cfg_read_byte(void) {
diff --git a/src/vnd_cfg.h b/src/vnd_cfg.h
index 82736fb..f5ba945 100644
--- a/src/vnd_cfg.h
+++ b/src/vnd_cfg.h
@@ -51,6 +51,7 @@ enum cfg_resp {
     cfg_resp_badarg     = 0x04
 };
 
+void    vnd_cfg_set_itf_num(int itf);
 uint8_t vnd_cfg_read_byte (void);
 void    vnd_cfg_drop_incoming(void);
 void    vnd_cfg_write_flush(void);