Reach level 44 by week 12
Here's the math: the total gem reward from upgrading to level 44 is 3,800 gems. Add 1,200 gem bonus from playing 84 consecutive days (=7 days * 12 weeks), you will have 5,000 gems enough to buy a VG. (If you can find some voucher codes that give away free gems, then you can achieve this even faster.)
Next, here's how to reach level 44 in 3 months without using a single gem.
Step 1. Use the cempire.org guide to clear the island (3-5 weeks). You will be about level 32.
Step 2. Play 12 horseback adventures (7,230 XP) with 3 normal generals to reach level 36. (~2 weeks)
Step 3. Play 29 Nords (27,530 XP) with 3 normal generals to reach level 44. (4-6 weeks)
I'll not give the specifics of each step because I don't want to spoil your fun. But if you PM or ingame mail me, I'll be happy to share the details.
Final note: This is far from easy because you will have to replace 400-600 troops (the effect of no fast gen) for each adventure and generate enough income to buy these adventures in TO. But if you design your economy efficiently, this is certainly doable. I hesitated posting this strategy for a while because it may create more demand for these adventures and increase the prices.
Background: I was not fortunate enough to start this game before the Easter event where one can get a free VG. But thanks to BB, with the level reward adjustment and 7-day gem bonus increase on 6/13/2013, one can now get a free VG with some planning and discipline. I'm not suggesting everyone should do this because without paying customers, some of us can't enjoy this great game for free.