Having an interface defined in the system with a longer name like vlan1000 makes xentop fail with the following message:

dom0-2:~ # xentop
*** buffer overflow detected ***: terminated
Aborted (core dumped)

I tracked this down using gdb showing a back trace

(gdb) bt
#0  0x00007ffff7862c6b in raise () from /lib64/libc.so.6
#1  0x00007ffff7864305 in abort () from /lib64/libc.so.6
#2  0x00007ffff78a8a97 in __libc_message () from /lib64/libc.so.6
#3  0x00007ffff7940812 in __fortify_fail () from /lib64/libc.so.6
#4  0x00007ffff793ec40 in __chk_fail () from /lib64/libc.so.6
#5  0x00007ffff793e3c2 in __strcpy_chk () from /lib64/libc.so.6
#6  0x00007ffff7fb3adb in strcpy (__src=<optimized out>, __dest=0x7fffffff6a00 "") at /usr/include/bits/string_fortified.h:90
#7  parseNetDevLine (rxFifo=0x0, rxFrames=0x0, rxComp=0x0, rxMcast=0x0, txFifo=0x0, txColls=0x0, txCarrier=0x0, txComp=0x0, txDrops=<synthetic pointer>,
    txErrs=<synthetic pointer>, txPackets=<synthetic pointer>, txBytes=<synthetic pointer>, rxDrops=<synthetic pointer>, rxErrs=<synthetic pointer>,
    rxPackets=<synthetic pointer>, rxBytes=<synthetic pointer>, iface=0x7fffffff6a00 "",
    line=0x7fffffff6a40 "vlan1000: 741038188 1521283    0    0    0     0          0         0 292284847 1305567    0    0    0     0       0          0\n")
    at xenstat_linux.c:233
#8  xenstat_collect_networks (node=0x555555584550) at xenstat_linux.c:322
#9  0x00007ffff7fb22d1 in xenstat_get_node (handle=<optimized out>, flags=15) at xenstat.c:245
#10 0x00005555555589f4 in ?? ()
#11 0x0000555555557337 in ?? ()
#12 0x00007ffff784d24d in __libc_start_main () from /lib64/libc.so.6
#13 0x000055555555772a in ?? ()

and finally fixed the problem by increasing the string lengths in xen-4.17.1-testing/tools/libs/stat/xenstat_linux.c function  int xenstat_collect_networks(xenstat_node * node) like so:

--- xen-4.17.1-orig/tools/libs/stat/xenstat_linux.c     2023-05-02 14:13:40.000000000 +0200
+++ xen-4.17.1-testing/tools/libs/stat/xenstat_linux.c  2023-06-25 13:28:19.328212646 +0200
@@ -272,7 +272,7 @@
 {
        /* Helper variables for parseNetDevLine() function defined above */
        int i;
-       char line[512] = { 0 }, iface[16] = { 0 }, devBridge[16] = { 0 }, devNoBridge[17] = { 0 };
+       char line[512] = { 0 }, iface[24] = { 0 }, devBridge[24] = { 0 }, devNoBridge[24] = { 0 };
        unsigned long long rxBytes, rxPackets, rxErrs, rxDrops, txBytes, txPackets, txErrs, txDrops;


I think naming VLAN device vlan1000 for VLAN 1000 is not to strange and should be handled okay. Not sure though why a string of 16 turns out to be too short but I figured out that extending this with 8 bytes did the job.