I'll leave a few recommendations here instead, just in case someone else wants something similar one day. As these restaurants tend to be very small I'll also give the instagram link in case you want to make a booking. First recommendation would be Kuma No Yakitori on Soi 47 in Thonglor area. There's a lot of competition for good Japanese food in this neighbourhood and standards are generally good so just exploring Thonglor and seeing what you fancy isn't a bad idea but this place has still been worth repeat visits despite need to be booked in advance as it's tiny,  https://www.instagram.com/kumanoyakitori_bkk/?hl=en In Ekkamai I'd recommend Tori Tama and Torisawa 22, both are on Soi 63. Torisawa is my current favourite but again it's a tiny place that needs booking in advance. A small group of us had a memorable but very drunken meal there last time around so I'm a little bit embarrassed to return but when next in Bangkok the food will be worth dealing with that. https://www.instagram.com/toritamabangkok/?hl=en https://www.instagram.com/torisawa22_bkk/?hl=en The streets around Ekkamai have plenty of other good options too. The area around Phrom Phong and Soi 33 is also good for Japanese bars and restaurants but my experiences have been more mixed than a bit further up Sukhumvit. Maybe it's just bad luck because I've picked at random when going to 7 Heaven BJ bar nearby (worth the trip in itself btw) or other similar dens if iniquity. One place that I like a lot in that area though is Unatoto. I've just about sneaked it onto this list as it's not a place for skewers, it's a casual unagi/eel restaurant but at least they're still grilling stuff so I'm happy to cheat a bit. It's good value and very busy but even if there's a queue the wait isn't normally bad.  Overall there's plenty of decent Japanese food in BKK. As a result it's probably the cuisine I eat most when wanting a change from Thai and it's rarely been bad even when it's places I wouldn't bother returning to - mostly due to the competition on offer nearby. If you see a random Izakaya that looks busy, it'll be good. 

if you wouldnt mind sending a pm I would love that. Especially if its reachable by BTS I don't mind going quite a ways. Its more the places that require a car where I might get stuck in traffic that is not worth my time because I'm coming on such a short stay. Thanks!

nice man thanks, gives me an excuse to go down there and have a drink at Vesper again!

No recommendations, but Thaniya Road in Patpong, right next to the Sala Daeng BTS station has a large number of Japanese restaurants. At night, there are a number of clubs with attractive women lining the streets. IIRC, this area caters to Japanese businessmen. 

The best and most varied/specialised Japanese food in Bangkok can be found a little further up Sukhumvit from Nana in the Phrom Phong, Thonglor and Ekkamai neighbourhoods. It's only 2-4 stops on the BTS or a fairly short walk to the area around Soi 33 especially while the weather is cooler. As a rule, you can get okish Japanese food in Nana, better around Asoke and it improves as you get further up Sukhumvit where there's a large Japanese influence particularly between Phrom Phong and towards Thonglor where there's loads of different izakayas of various types. It's well worth exploring this area and seeing what appeals to you if you're really looking for the good stuff. I can recommend a few places in that area if it's not too far as it seems like you're interested in the better options.