I'm working on something related to this. I have a number of different solutions that work better in different scenarios.
To help you best we can look at your project two different ways, each having a different set of initial questions and each leading to different discussions.
A. Doing it the way you're doing it.
Question 1: What device is the source of PAL signal?
Question 2. What is the output display that you want to drive with the VGA signal?
B. A more abstract look at your project.
Question: What is the end goal that you want to achieve with the solution? What's it used for?
------------------------
Some facts to help you a bit:
PAL (and NTSC) are both television standard signals. Both have about 15KHz horizontal refresh rate, while VGA specification requires 30KHz+.
Although PAL is often said to have a vertical resolution of 576 lines, the signal is interlaced, and each field at 50Hz is only 288 lines vertical resolution, so it's best looked at like this, 576 lines at 25Hz or 288 at 50Hz. To get 576 in a single refresh cycle the signal has to be deinterlaced. This is something that you don't mention above, whether you already deinterlace your signal or not. Depending on your signal source deinterlacing technique can vary, so that's something to keep in mind. If you work with 576 line frames at 25Hz, you don't necessarily have to pick 100Hz refresh rate for VGA like you said above. 75 is a multiple of 25, so 75Hz can be a good option, and it has the advantage that LCD monitors can sync to that, while they won't sync to 100Hz.
Horizontal resolution if PAL, there's an interesting topic. PAL being a CRT TV signal doesn't really define a specific horizontal resolution. You can divide a scanline into as many segments (or pixels) as you want. 720 is just a number, it has no real relation to PAL. Are you sure that 720 for horizontal resolution is the most optimal for your application? It's an analog signal, a pixel is a digital concept. Pixels don't really exist in PAL. So that's something you can perhaps reconsider.
Generally LCD monitors with VGA input will sync to 60 to 75Hz vertical refresh, rarely anything outside that range.
Generally VGA monitors will sync to an entire range of vertical refresh rates, they don't have to be one of the common values like 60, 70, 75. You can have (as an example) a vertical refresh of 62.5Hz, and that's acceptable for a VGA monitor. In fact often CRT VGA monitors will work with 50Hz vertical refresh, the flicker will be strong though, but that depends on the phosphors of a particular CRT. While I said you can do 50Hz with a VGA CRT, you still have to obey the 30KHz+ specification for horizontal refresh.
VGA LCD monitors will not sync to 50Hz vertical refresh because LCD monitor manufacturers are just being jerks, there is no technical reason why they can't work with that refresh rate. There is actually a way to get around this, but it's a rather extreme measure.
VGA signal also technically has no horizontal resolution. But to show picture on an LCD, which *does* have a fixed horizontal resolution the LCD monitor does clock-from-data recovery on your VGA signal to get the pixel clock and then buffer the frame and then scale it (if needed) to the native resolution of the LCD. The scaler chip in LCD monitor usually only works with a few common input resolutions.
Unlike LCD however, a CRT monitor will accept any horizontal resolution you like, so that's something to keep in mind. This fact becomes useful when you add pixel clock constraints into your project.
So to make your PAL signal into a VGA-compatible one it's mostly the task of converting that 15KHz horizontal refresh into something 30KHz+. And there's more than one way to do it.
Because VGA is a computer monitor signal standard I will give you a real world computer example. In the past 320x200 was a common computer resolution, and when VGA standard was introduced there was a need to display it. The problem with 320x200 modes is that they also had 15KHz horizontal refresh rate, but VGA specification requires 30KHz+ horizontal refresh rate. The solution here was double scan. The VGA controller would repeat each scanline of the 200 to paint 400 lines on the monitor, to achieve the necessary 30KHz horizontal refresh rate. It's just an example for you to examine.
If you haven't abandoned this thread yet, let me know if you want to discuss further. I work on many projects like this.