Log:

(all $variables are set correctly)

ffmpeg \
   -init_hw_device vaapi=amd:/dev/dri/renderD128 \
   -hwaccel vaapi -hwaccel_device amd \
   -c:v vp9 -hwaccel_output_format vaapi -an \
   -extra_hw_frames $lad \
   -i orig-vp9.mp4 \
   -frames:v $numframes \
   -c:v hevc_vaapi \
   -preset veryslow \
   -profile:v 1 \
   -low_power false \
   -async_depth $cores \
   -b:v $bitrate \
   -maxrate $((2 * $bitrate)) \
   -bufsize $((4 * $bitrate)) \
   -fps_mode passthrough -y \
   hevc.mp4
   # Adding this below line (uncommented) causes the command to fail citing
   # "Cannot allocate memory"
   # -vf 'scale_vaapi=-1:720' \

Log of failure:
ffmpeg version n7.0.1 Copyright (c) 2000-2024 the FFmpeg developers
  built with gcc 14.1.1 (GCC) 20240522
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --enable-cuda-llvm --enable-lto --enable-fontconfig --enable-frei0r --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libdav1d --enable-libdrm --enable-libdvdnav --enable-libdvdread --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libharfbuzz --enable-libiec61883 --enable-libjack --enable-libjxl --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libplacebo --enable-libpulse --enable-librav1e --enable-librsvg --enable-librubberband --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpl --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-libzimg --enable-nvdec --enable-nvenc --enable-opencl --enable-opengl --enable-shared --enable-vapoursynth --enable-version3 --enable-vulkan
  libavutil      59.  8.100 / 59.  8.100
  libavcodec     61.  3.100 / 61.  3.100
  libavformat    61.  1.100 / 61.  1.100
  libavdevice    61.  1.100 / 61.  1.100
  libavfilter    10.  1.100 / 10.  1.100
  libswscale      8.  1.100 /  8.  1.100
  libswresample   5.  1.100 /  5.  1.100
  libpostproc    58.  1.100 / 58.  1.100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x5784a5224b40] stream 0, timescale not set
[vp9 @ 0x5784a518c900] Requested reference 1 not available
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x5784a5224b40] Could not find codec parameters for stream 2 (Video: vp9 (Profile 0), none): unspecified size
Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (5000000) options
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'orig-vp9.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2mp41
    encoder         : Lavf59.16.100
  Duration: 07:51:03.40, start: 0.000000, bitrate: 4081 kb/s
  Stream #0:0[0x1](eng): Video: vp9 (Profile 0) (vp09 / 0x39307076), yuv420p(tv, bt470bg/bt709/bt709), 1920x1080, 3960 kb/s, SAR 1:1 DAR 16:9, 60 fps, 60 tbr, 16k tbn (default)
      Metadata:
        handler_name    : VideoHandler
        vendor_id       : [0][0][0][0]
  Stream #0:1[0x2](eng): Audio: opus (Opus / 0x7375704F), 48000 Hz, mono, fltp, 101 kb/s (default)
      Metadata:
        handler_name    : SoundHandler
        vendor_id       : [0][0][0][0]
  Stream #0:2[0x0]: Video: vp9 (Profile 0), none, 90k tbr, 90k tbn (attached pic)
[out#0/mp4 @ 0x5784a5188680] Codec AVOption preset (Encoding preset) has not been used for any stream. The most likely reason is either wrong type (e.g. a video option with no video streams) or that it is a private option of some encoder which was not actually used for any stream.
Stream mapping:
  Stream #0:0 -> #0:0 (vp9 (native) -> hevc (hevc_vaapi))
Press [q] to stop, [?] for help
[hevc_vaapi @ 0x5784a518c900] No quality level set; using default (25).
[hevc_vaapi @ 0x5784a518c900] Driver does not support some wanted packed headers (wanted 0xd, found 0x1).
Output #0, mp4, to 'hevc.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2mp41
    encoder         : Lavf61.1.100
  Stream #0:0(eng): Video: hevc (Main) (hev1 / 0x31766568), vaapi(tv, bt470bg/bt470bg/smpte170m, progressive), 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 60 fps, 15360 tbn (default)
      Metadata:
        handler_name    : VideoHandler
        vendor_id       : [0][0][0][0]
        encoder         : Lavc61.3.100 hevc_vaapi
[vf#0:0 @ 0x5784a51440c0] Error while filtering: Cannot allocate memory
[vf#0:0 @ 0x5784a51440c0] Task finished with error code: -12 (Cannot allocate memory)
[vf#0:0 @ 0x5784a51440c0] Terminating thread with return code -12 (Cannot allocate memory)
[out#0/mp4 @ 0x5784a5188680] video:11KiB audio:0KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: 22.129284%
frame=   10 fps=0.0 q=-0.0 Lsize=      13KiB time=00:00:00.16 bitrate= 644.8kbits/s speed=2.34x    
Conversion failed!