Honestly, don't sweat the V90 thing too much if you're just doing travel vlogging. I have used Sony Tough V60 128GB SDXC cards for years with my a7 IV and they handle 4K 60fps 10-bit 4:2:2 without a single hiccup. Those V90 cards are overkill unless you are doing crazy high-bitrate stuff or professional production work. Save your cash for the trip. If you want a solid alternative to Sony, I have had zero issues with ProGrade Digital V60 128GB SDXC UHS-II cards lately, and they are usually a bit cheaper. Just make sure you are getting the V60 rating minimum so your buffer doesnt fill up during recording. You will be totally fine with these, just grab two and you are set for Japan.

Building on the earlier suggestion, I totally get the stress. I bought a bunch of cheap cards for my first trip and had half my files end up corrupted. Total nightmare. Honestly, for the a7 IV, just dont go for the absolute cheapest generic ones you find on Amazon. If you want to save cash but keep things safe, I have been running these lately without any complaints:

• SanDisk Extreme Pro 128GB SDXC UHS-I
• Good for photos and lower bitrate stuff, but be careful with 60fps, it might chug.
• Delkin Devices Power 128GB SDXC UHS-II
• This is my go-to for video. It is way more reliable than the no-name brands and handles the 4K 60fps write speeds like a champ. Make sure to format the card in the camera right before you start shooting each day. It sounds obvious, but I have forgotten that step and had weird glitches happen mid-shoot. Also, bring a couple smaller capacity cards instead of one giant one. If you lose or break a card in Japan, you wont lose your entire trip footage. I keep mine in a hard plastic case because I am clumsy. Just stay away from anything that doesnt list a V60 rating minimum, or you are definitely gonna see that recording icon start blinking at you.

Jumping in here because the storage situation with the a7 IV is actually a bit of a headache with the bitrates. If you are worried about reliability during your trip, honestly, look into ProGrade Digital V90 128GB SDXC UHS-II. I know you mentioned wanting to save cash, but if you are shooting 4K 60fps at high bitrates, the write speeds on a V90 card give you that extra buffer so the camera buffer doesn't clear slowly. It just feels safer when you are traveling and don't want to risk losing files because the card choked. Another solid alternative that has treated me well is the Lexar Professional 1667x 128GB SDXC UHS-II V60. It is definitely cheaper, but keep in mind that with V60 cards, you might hit a wall if you ever decide to push the camera to its absolute limits with the highest ALL-I codecs. For general travel stuff, it works totally fine and I have never had a write error. My advice is to stick with established brands like ProGrade or Lexar because they have better longevity with the read/write cycles you get from 10-bit footage. Whatever you do, dont cheap out on some random generic brand, or you will definitely regret it when you get home and realize your files are corrupted. Just grab a couple of decent mid-range cards so you have a backup.