Welcome to the Strelets Forum. Please feel free to discuss any aspect of 1/72 scale plastic figures, not simply Strelets. If you have any questions about our products then we will answer them here.
If thinning acrylics with water has not worked for you, try like I said previously, use washing up liquid instead. It tends to prevent the acrylic from drying too quickly, but also helps the paint's viscosity (I think thats the word?!), so that it is more "spreadable" without turning the paint too watery, which sounds like your issue when thinning.
Put your paint on the palette first then squeeze a couple of drops of washing up liquid on the top of the paint, (for around a small coin worth of paint deposit, I generally use 2-3 drops of washing liquid). But have an experiment with the amount until you get it how you like.
Mix the washing up liquid and paint together on your palette so its all fully absorbed into the paint. I just use the handle end of an old paint brush.
If you have an empty Vallejo acrylic bottle spare, wash it out thoroughly (including the nozzle and cap), remove the nozzle as mentioned before (either by getting under the seam and lifting or using pliers/forceps to pull it out) & put fill that up with the washing up liquid. That way due to the dropper function of the bottle you have better control of how many drops you want to put in the paint.
Also it was Game Colour I found that was more hit & miss on consistency, not Model Colour. Model Colour I have been pretty happy with. Both the product & the choice of colours available.
If you still have issues with the Vallejo range despite trying all what me and others have mentioned, my advice is try the Army Painter range. When I have had to use them, due to my prefered being out of stock, I have found them to be a thinner consistency in general compared to Vallejo. You will probably find that at least 2 coats will be needed. Maybe some of their colours will be better than others on that front. But some browns and pale colours of theirs I have found to be indeed thin.
If there are Vallejo colours you are happy with, I think somewhere online I seen a conversion chart between Vallejo, Army Painter & Citadel etc. So you can find the nearest match to the colour when switching over.
try the Army Painter range. When I have had to use them, due to my preferred being out of stock, I have found them to be a thinner consistency in general compared to Vallejo..
Roger is correct on this. I do like & use the Army Painter range as well but they need "extra" shaking - note, they now come with a ball bearing added to the dropper bottle to help with this.
I must admit I'm intrigued by Roger's fairy liquid hack. I don't really need it but I'm going to give it a go out of interest. You are never to old to learn new tricks!